Fix color and depth buffer cache for windows that change size.  Fix documentation referring to "singleton" buffer manager instead of per-context.
Fix color and depth buffer cache for windows that change size.  Fix
documentation referring to "singleton" buffer manager instead of per-context.

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