gdk: Make gdk_window_begin_draw_frame() take a draw context

gdk: Make gdk_window_begin_draw_frame() take a draw context

... instead of a gl context.

This requires some refactoring in the way we mark the shared context as
drawing: We now call begin_frame/end_frame() on it and ignore the call
on the main context.
Unfortunately we need to do this check in all vfuncs, which sucks. But I
haven't found a better way.