MainFrm: Consistently use IsD3DFullScreenMode() to check if D3DFullscreen is active.

This is more consistent and allows us to easily change the conditions in a central location.
MainFrm: Remove redundant OSD message hiding.

This is already done in OnResetDevice().
SyncRenderer: Add support for ID3DFullscreenControl interface.
Call ToggleD3DFullscreen from GraphThread if it is active.

This prevents the MainThread from manipulating the fullscreen window which it does not own.