Fix win32 expose context switch.
Fix win32 expose context switch.

niklaskorz/pyglet,adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,niklaskorz/pyglet,seeminglee/pyglet64,seeminglee/pyglet64,seeminglee/pyglet64,niklaskorz/pyglet,adamlwgriffiths/Pyglet,niklaskorz/pyglet