FFXIV depends on this. It calls SetFullscreenState when it receives WM_WINDOWPOSCHANGED. Through luck the recursion aborts after two calls. The user-visible bug is that after leaving fullscreen, the game's window is still TOPMOST because we overwrite the stored pre-fullscreen window flags.
-- v3: dxgi/tests: Test nested SetFullscreenState from the same thread. dxgi/tests: Test nested fullscreen application from different thread. dxgi: Catch nested SetFullscreenState invocations. dxgi: Unlock the wined3d mutex after storing the new target. dxgi/tests: Run test_swapchain_window_messages on d3d12. dxgi/tests: Run test_resize_target_wndproc on d3d12 too. dxgi/tests: Fix UnregisterClass call in test_resize_target_wndproc.