Add a new hint SDL_HINT_VIDEO_WINDOW_SHARE_PIXEL_FORMAT that allows SDL_CreateWindowFrom() to set the pixel format of another SDL_Window (and also will set the SDL_WINDOW_OPENGL flag on the window created with SDL_CreateWindowFrom()).

The reasoning behind this change is that source2 in -tools mode has a single OpenGL context that is used with multiple different windows.  Some of those windows are created outside the engine (i.e. with Qt) and therefore we need to use SDL_CreateWindowFrom() to get an SDL_Window for those.  The requirement for sharing an OpenGL context across multiple different windows is that each window has the same pixel format.  To facilitate this, I now set SDL_HINT_VIDEO_WINDOW_SHARE_PIXEL_FORMAT for the main window before calling SDL_CreateWindowFrom().  When I do this, SDL_CreateWindowFrom() will:

1. Set the pixel format of the returned window to the same pixel format as this SDL_Window passed in with the hint
2. The flag SDL_WINDOW_OPENGL will be set on the new window so it can be used for OpenGL rendering.

I only currently implemented this for Win32/WGL so implementing it for other platforms (i.e. X11) remains a TODO.

CR: SamL

Some pseudocode that shows how this is used in Source2:

HWND hExternalHwnd; // HWND that was established outside of SDL

// Create main window (happens inside platwindow.cpp)
SDL_Window *mainWindow = SDL_CreateWindow( , SDL_WINDOW_OPENGL .. );
// Create GL context, happens inside rendersystemgl
SDL_GLContext onlyContext = SDL_GL_CreateContext( mainWindow );

// Now I need to create another window from hEternalHwnd for my swap chain that will have the same pixel format as mainWindow, so set the hint
SDL_SetHint( SDL_HINT_VIDEO_WINDOW_SHARE_PIXEL_FORMAT, CFmtStr( %p, mainWindow) );

// Create the secondary window.  This returned window will have SDL_WINDOW_OPENGL set and share a pixel format with mainWindow from the hint
SDL_Window *secondaryWindow = SDL_CreateWindowFrom( hExternalHwnd );

// To render to the main window:
SDL_GL_MakeCurrent( mainWindow, onlyContext );
// Do some rendering to main window

// To render to the secondary window:
SDL_GLMakeCurrent( secondaryWindow, onlyContext );
// Do some rendering to secondary window

include/SDL_hints.h

@@ -328,6 +328,25 @@ extern "C" {
 */
 #define SDL_HINT_VIDEO_WIN_D3DCOMPILER              "SDL_VIDEO_WIN_D3DCOMPILER"
 
+/**
+*  \brief  A variable that is the address of another SDL_Window* (as a hex string formatted with "%p").
+*  
+*  If this hint is set before SDL_CreateWindowFrom() and the SDL_Window* it is set to has
+*  SDL_WINDOW_OPENGL set (and running on WGL only, currently), then two things will occur on the newly 
+*  created SDL_Window:
+
+*  1. Its pixel format will be set to the same pixel format as this SDL_Window.  This is
+*  needed for example when sharing an OpenGL context across multiple windows.
+*
+*  2. The flag SDL_WINDOW_OPENGL will be set on the new window so it can be used for
+*  OpenGL rendering.
+*
+*  This variable can be set to the following values:
+*    The address (as a string "%p") of the SDL_Window* that new windows created with SDL_CreateWindowFrom() should
+*    share a pixel format with.
+*/
+#define SDL_HINT_VIDEO_WINDOW_SHARE_PIXEL_FORMAT    "SDL_VIDEO_WINDOW_SHARE_PIXEL_FORMAT"
+
 /**
  *  \brief  An enumeration of hint priorities
  */

src/video/windows/SDL_windowsopengl.c

@@ -759,6 +759,26 @@ WIN_GL_DeleteContext(_THIS, SDL_GLContext context)
     _this->gl_data->wglDeleteContext((HGLRC) context);
 }
 
+
+SDL_bool
+WIN_GL_SetPixelFormatFrom(_THIS, SDL_Window * fromWindow, SDL_Window * toWindow)
+{
+    HDC hfromdc = ((SDL_WindowData *) fromWindow->driverdata)->hdc;
+    HDC htodc = ((SDL_WindowData *) toWindow->driverdata)->hdc;
+    BOOL result;
+
+    /* get the pixel format of the fromWindow */
+    int pixel_format = GetPixelFormat(hfromdc);
+    PIXELFORMATDESCRIPTOR pfd;
+    SDL_memset(&pfd, 0, sizeof(pfd));
+    DescribePixelFormat(hfromdc, pixel_format, sizeof(pfd), &pfd);
+
+    /* set the pixel format of the toWindow */
+    result = SetPixelFormat(htodc, pixel_format, &pfd);
+
+    return result ? SDL_TRUE : SDL_FALSE;
+}
+
 #endif /* SDL_VIDEO_OPENGL_WGL */
 
 #endif /* SDL_VIDEO_DRIVER_WINDOWS */

src/video/windows/SDL_windowsopengl.h

@@ -64,6 +64,7 @@ extern int WIN_GL_GetSwapInterval(_THIS);
 extern void WIN_GL_SwapWindow(_THIS, SDL_Window * window);
 extern void WIN_GL_DeleteContext(_THIS, SDL_GLContext context);
 extern void WIN_GL_InitExtensions(_THIS);
+extern SDL_bool WIN_GL_SetPixelFormatFrom(_THIS, SDL_Window * fromWindow, SDL_Window * toWindow);
 
 #ifndef WGL_ARB_pixel_format
 #define WGL_NUMBER_PIXEL_FORMATS_ARB   0x2000

src/video/windows/SDL_windowswindow.c

@@ -32,6 +32,7 @@
 
 #include "SDL_windowsvideo.h"
 #include "SDL_windowswindow.h"
+#include "SDL_hints.h"
 
 /* Dropfile support */
 #include <shellapi.h>
@@ -337,6 +338,31 @@ WIN_CreateWindowFrom(_THIS, SDL_Window * window, const void *data)
     if (SetupWindowData(_this, window, hwnd, SDL_FALSE) < 0) {
         return -1;
     }
+
+#if SDL_VIDEO_OPENGL_WGL
+    {
+        const char *hint = SDL_GetHint(SDL_HINT_VIDEO_WINDOW_SHARE_PIXEL_FORMAT);
+        if (hint) {
+            // This hint is a pointer (in string form) of the address of
+            // the window to share a pixel format with
+            SDL_Window *otherWindow = NULL;
+            SDL_sscanf(hint, "%p", (void**)&otherWindow);
+
+            // Do some error checking on the pointer
+            if (otherWindow != NULL && otherWindow->magic == &_this->window_magic)
+            {
+                // If the otherWindow has SDL_WINDOW_OPENGL set, set it for the new window as well
+                if (otherWindow->flags & SDL_WINDOW_OPENGL)
+                {
+                    window->flags |= SDL_WINDOW_OPENGL;
+                    if(!WIN_GL_SetPixelFormatFrom(_this, otherWindow, window)) {
+                        return -1;
+                    }
+                }
+            }
+        }
+    }
+#endif
     return 0;
 }