gdk: Move begin/end_frame() functions

gdk: Move begin/end_frame() functions

As they require a draw context and the draw context is already bound to
the surface, it makes much more sense and reduces abiguity by moving
these APIs to the draw context.

As a side effect, we simplify GdkSurface APIs to a point where
GdkSurface now does not concern itself with drawing anymore at all,
apart from being the object that creates draw contexts.