Engine: also release render targets when switching in fullscreen

Unfortunately, it seems like Direct3D also must reset device when switching in back into fullscreen (after restoring minimized window).

Judging by the tests, when switching back to fullscreen (after alt+tabbing out), there may be 2 window events:
* window size changed
* window focus received

The former sometimes either is not received, or received too late(?), but latter is guaranteed.

Engine/ac/game.cpp

@@ -1332,6 +1332,10 @@ void display_switch_in_resume()
     }
     video_resume();
 
+    // release render targets if switching back to the full screen mode;
+    // unfortunately, otherwise Direct3D fails to reset device when restoring fullscreen.
+    if (gfxDriver && gfxDriver->GetDisplayMode().IsRealFullscreen())
+        release_drawobj_rendertargets();
     // clear the screen if necessary
     if (gfxDriver && gfxDriver->UsesMemoryBackBuffer())
         gfxDriver->ClearRectangle(0, 0, game.GetGameRes().Width - 1, game.GetGameRes().Height - 1, nullptr);