WinRT: added Win10/UWP (Universal Windows Platform) support

"UWP" appears to be Microsoft's new name for WinRT/Windows-Store APIs.

This set of changes updates SDL's WinRT backends to support the Win10 flavor
of WinRT.  It has been tested on Win10 on a desktop.  In theory, it should
also support Win10 on other devices (phone, Xbox One, etc.), however further
patches may be necessary.

This adds:
- a set of MSVC 2015 project files, for use in creating UWP apps
- modifications to various pieces of SDL, in order to compile via MSVC 2015 +
  the Win10 API set
- enables SDL_Window resizing and programmatic-fullscreen toggling, when using
  the WinRT backend
- WinRT README updates

src/video/SDL_video.c

@@ -1168,7 +1168,7 @@ SDL_UpdateFullscreenMode(SDL_Window * window, SDL_bool fullscreen)
         window->last_fullscreen_flags = window->flags;
         return 0;
     }
-#elif __WINRT__
+#elif __WINRT__ && (NTDDI_VERSION < NTDDI_WIN10)
     /* HACK: WinRT 8.x apps can't choose whether or not they are fullscreen
        or not.  The user can choose this, via OS-provided UI, but this can't
        be set programmatically.
@@ -1405,7 +1405,7 @@ SDL_CreateWindow(const char *title, int x, int y, int w, int h, Uint32 flags)
         return NULL;
     }
 
-#if __WINRT__
+#if __WINRT__ && (NTDDI_VERSION < NTDDI_WIN10)
     /* HACK: WinRT 8.x apps can't choose whether or not they are fullscreen
        or not.  The user can choose this, via OS-provided UI, but this can't
        be set programmatically.

src/video/winrt/SDL_winrtvideo.cpp

@@ -76,6 +76,8 @@ static void WINRT_VideoQuit(_THIS);
 
 /* Window functions */
 static int WINRT_CreateWindow(_THIS, SDL_Window * window);
+static void WINRT_SetWindowSize(_THIS, SDL_Window * window);
+static void WINRT_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display, SDL_bool fullscreen);
 static void WINRT_DestroyWindow(_THIS, SDL_Window * window);
 static SDL_bool WINRT_GetWindowWMInfo(_THIS, SDL_Window * window, SDL_SysWMinfo * info);
 
@@ -134,6 +136,8 @@ WINRT_CreateDevice(int devindex)
     device->VideoInit = WINRT_VideoInit;
     device->VideoQuit = WINRT_VideoQuit;
     device->CreateWindow = WINRT_CreateWindow;
+    device->SetWindowSize = WINRT_SetWindowSize;
+    device->SetWindowFullscreen = WINRT_SetWindowFullscreen;
     device->DestroyWindow = WINRT_DestroyWindow;
     device->SetDisplayMode = WINRT_SetDisplayMode;
     device->PumpEvents = WINRT_PumpEvents;
@@ -478,6 +482,9 @@ WINRT_CreateWindow(_THIS, SDL_Window * window)
 #endif
     }
 
+    /* Make note of the requested window flags, before they start getting changed. */
+    const Uint32 requestedFlags = window->flags;
+
 #if SDL_VIDEO_OPENGL_EGL
     /* Setup the EGL surface, but only if OpenGL ES 2 was requested. */
     if (!(window->flags & SDL_WINDOW_OPENGL)) {
@@ -548,15 +555,35 @@ WINRT_CreateWindow(_THIS, SDL_Window * window)
         SDL_SetMouseFocus(NULL);        // TODO: detect this
         SDL_SetKeyboardFocus(NULL);     // TODO: detect this
     } else {
-        /* WinRT apps seem to live in an environment where the OS controls the
+        /* WinRT 8.x apps seem to live in an environment where the OS controls the
            app's window size, with some apps being fullscreen, depending on
            user choice of various things.  For now, just adapt the SDL_Window to
            whatever Windows set-up as the native-window's geometry.
         */
         window->x = WINRT_DIPS_TO_PHYSICAL_PIXELS(data->coreWindow->Bounds.Left);
         window->y = WINRT_DIPS_TO_PHYSICAL_PIXELS(data->coreWindow->Bounds.Top);
+#if NTDDI_VERSION < NTDDI_WIN10
+        /* On WinRT 8.x / pre-Win10, just use the size we were given. */
         window->w = WINRT_DIPS_TO_PHYSICAL_PIXELS(data->coreWindow->Bounds.Width);
         window->h = WINRT_DIPS_TO_PHYSICAL_PIXELS(data->coreWindow->Bounds.Height);
+#else
+        /* On Windows 10, we occasionally get control over window size.  For windowed
+           mode apps, try this.
+        */
+        bool didSetSize = false;
+        if (!(requestedFlags & SDL_WINDOW_FULLSCREEN)) {
+            const Windows::Foundation::Size size(WINRT_PHYSICAL_PIXELS_TO_DIPS(window->w),
+                                                 WINRT_PHYSICAL_PIXELS_TO_DIPS(window->h));
+            didSetSize = data->appView->TryResizeView(size);
+        }
+        if (!didSetSize) {
+            /* We either weren't able to set the window size, or a request for
+               fullscreen was made.  Get window-size info from the OS.
+            */
+            window->w = WINRT_DIPS_TO_PHYSICAL_PIXELS(data->coreWindow->Bounds.Width);
+            window->h = WINRT_DIPS_TO_PHYSICAL_PIXELS(data->coreWindow->Bounds.Height);
+        }
+#endif
 
         WINRT_UpdateWindowFlags(
             window,
@@ -584,6 +611,35 @@ WINRT_CreateWindow(_THIS, SDL_Window * window)
     return 0;
 }
 
+void
+WINRT_SetWindowSize(_THIS, SDL_Window * window)
+{
+#if NTDDI_VERSION >= NTDDI_WIN10
+    SDL_WindowData * data = (SDL_WindowData *)window->driverdata;
+    const Windows::Foundation::Size size(WINRT_PHYSICAL_PIXELS_TO_DIPS(window->w),
+                                         WINRT_PHYSICAL_PIXELS_TO_DIPS(window->h));
+    data->appView->TryResizeView(size); // TODO, WinRT: return failure (to caller?) from TryResizeView()
+#endif
+}
+
+void
+WINRT_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display, SDL_bool fullscreen)
+{
+#if NTDDI_VERSION >= NTDDI_WIN10
+    SDL_WindowData * data = (SDL_WindowData *)window->driverdata;
+    if (fullscreen) {
+        if (!data->appView->IsFullScreenMode) {
+            data->appView->TryEnterFullScreenMode();    // TODO, WinRT: return failure (to caller?) from TryEnterFullScreenMode()
+        }
+    } else {
+        if (data->appView->IsFullScreenMode) {
+            data->appView->ExitFullScreenMode();
+        }
+    }
+#endif
+}
+
+
 void
 WINRT_DestroyWindow(_THIS, SDL_Window * window)
 {

src/video/winrt/SDL_winrtvideo_cpp.h

@@ -75,9 +75,9 @@ extern "C" Uint32 WINRT_DetectWindowFlags(SDL_Window * window);  /* detects flag
 #define WINRT_DISPLAY_PROPERTY(NAME) (Windows::Graphics::Display::DisplayProperties::NAME)
 #endif
 
-/* Converts DIPS to physical pixels */
-#define WINRT_DIPS_TO_PHYSICAL_PIXELS(DIPS) ((int)(0.5f + (((float)(DIPS) * (float)WINRT_DISPLAY_PROPERTY(LogicalDpi)) / 96.f)))
-
+/* Converts DIPS to/from physical pixels */
+#define WINRT_DIPS_TO_PHYSICAL_PIXELS(DIPS)     ((int)(0.5f + (((float)(DIPS) * (float)WINRT_DISPLAY_PROPERTY(LogicalDpi)) / 96.f)))
+#define WINRT_PHYSICAL_PIXELS_TO_DIPS(PHYSPIX)  (((float)(PHYSPIX) * 96.f)/WINRT_DISPLAY_PROPERTY(LogicalDpi))
 
 /* Internal window data */
 struct SDL_WindowData