video: Return the display ID when the window is fully enclosed

If the window was fully enclosed, GetDisplayForRect() would return the index of the display ID in the array instead of the display ID itself. Return the display ID itself.

src/video/SDL_video.c

@@ -1215,7 +1215,7 @@ static SDL_DisplayID GetDisplayForRect(int x, int y, int w, int h)
 
             /* Check if the window is fully enclosed */
             if (SDL_GetRectEnclosingPoints(&center, 1, &display_rect, NULL)) {
-                return i;
+                return display->id;
             }
 
             /* Snap window center to the display rect */