Engine: force gfxdriver to clear batches when releasing render targets

This is to ensure that no pointers or references to these render targets are left in gfx driver.
In the worst case, the game will skip 1 draw frame after resuming or switching display mode.

Engine/ac/draw.cpp

@@ -644,6 +644,7 @@ void release_drawobj_rendertargets()
         !gfxDriver->ShouldReleaseRenderTargets())
         return;
 
+    gfxDriver->ClearDrawLists(); // force clear to ensure nothing stays cached
     for (auto &tex : gui_render_tex)
     {
         if (tex)