Improve SDL_CreateWindow() fullscreen support on Windows

.. and maybe other platforms as well (though X11 was not affected)?

The issue was that passing a higher resolution than the current desktop
resolution to SDL_CreateWindow() with SDL_WINDOW_FULLSCREEN didn't switch
to that resolution (even though it did switch to lower resolutions).

When creating a fullscreen window, window->fullscreen wasn't even set
at all (only zeroed out), setting it only happened if the user explicitly
called SDL_SetWindowDisplayMode(). So without that, SDL_CreateWindow()
-> SDL_UpdateFullscreenMode() -> SDL_GetWindowDisplayMode() used the
resolution from window->windowed.w/h which were limited to the desktop size
due to some weird combination of WIN_AdjustWindowRectWithStyle() and
WIN_WindowProc() being called after a call to SetWindowPos().

fixes #3313

src/video/SDL_video.c

@@ -1598,6 +1598,22 @@ SDL_CreateWindow(const char *title, int x, int y, int w, int h, Uint32 flags)
         displayIndex = SDL_GetIndexOfDisplay(display);
         SDL_GetDisplayBounds(displayIndex, &bounds);
 
+        /* for real fullscreen we might switch the resolution, so get width and height
+         * from closest supported mode and use that instead of current resolution
+         */
+        if ((flags & SDL_WINDOW_FULLSCREEN_DESKTOP) != SDL_WINDOW_FULLSCREEN_DESKTOP
+              && (bounds.w != w || bounds.h != h)) {
+            SDL_DisplayMode fullscreen_mode, closest_mode;
+            SDL_zero(fullscreen_mode);
+            fullscreen_mode.w = w;
+            fullscreen_mode.h = h;
+            if(SDL_GetClosestDisplayModeForDisplay(display, &fullscreen_mode, &closest_mode) != NULL) {
+                bounds.w = closest_mode.w;
+                bounds.h = closest_mode.h;
+            }
+        }
+        window->fullscreen_mode.w = bounds.w;
+        window->fullscreen_mode.h = bounds.h;
         window->x = bounds.x;
         window->y = bounds.y;
         window->w = bounds.w;