Fix OpenGL VSync
RenderFrame: Set the background to black.
Merge pull request #537 from Armada651/master

RenderFrame: Set the background to black.
Merge pull request #533 from Anti-Ultimate/master

Fix OpenGL VSync
Make the emulation stop asynchronous to prevent deadlocks.

This may expose bugs which relied on the Main Thread to be suspended in the stopping state.
Frame: OnClose now handles asynchronous behaviour of DoStop().
Core: Return CORE_STOPPING even if hardware is already uninitialized.
FrameTools: Update GUI and disable Play button when in stopping state.