video: Store the preferred fullscreen display in a property for sdl2-compat

sdl2-compat will call SDL_GetDisplayForWindow() when querying the display to use for fullscreen, which won't always be correct if the backend can't actually reposition the window. When calling this function, get the ideal fullscreen display and store it in a property for retrieval by sdl2-compat.

Author: Kontrabant

src/video/SDL_video.c

@@ -1731,12 +1731,25 @@ SDL_VideoDisplay *SDL_GetVideoDisplayForFullscreenWindow(SDL_Window *window)
     return SDL_GetVideoDisplay(displayID);
 }
 
+#define SDL_PROP_SDL2_COMPAT_WINDOW_PREFERRED_FULLSCREEN_DISPLAY "sdl2-compat.window.preferred_fullscreen_display"
+
 SDL_DisplayID SDL_GetDisplayForWindow(SDL_Window *window)
 {
     SDL_DisplayID displayID = 0;
 
     CHECK_WINDOW_MAGIC(window, 0);
 
+    /* sdl2-compat calls this function to get a display on which to make the window fullscreen,
+     * so pass it the preferred fullscreen display ID in a property.
+     */
+    SDL_PropertiesID window_props = SDL_GetWindowProperties(window);
+    SDL_VideoDisplay *fs_display = SDL_GetVideoDisplayForFullscreenWindow(window);
+    if (fs_display) {
+        SDL_SetNumberProperty(window_props, SDL_PROP_SDL2_COMPAT_WINDOW_PREFERRED_FULLSCREEN_DISPLAY, fs_display->id);
+    } else {
+        SDL_ClearProperty(window_props, SDL_PROP_SDL2_COMPAT_WINDOW_PREFERRED_FULLSCREEN_DISPLAY);
+    }
+
     // An explicit fullscreen display overrides all
     if (window->flags & SDL_WINDOW_FULLSCREEN) {
         displayID = window->current_fullscreen_mode.displayID;