No visible changes as GL rendering is disabled at the moment.

What was done:

1. Move window->invalidate_for_new_frame to glcontext->begin_frame
This moves the code to where it is used (the GLContext) and prepares it
for being called where it is used when actually beginning to draw the
frame.

2. Get rid of buffer-age usage
We want to let the application render directly to the backbuffer.
Because of that, we cannot make any assumptions about the contents the
application renders outside the clip area.
In particular GskGLRenderer renders random stuff there but not actual
contents.

3. Pass the actual GL context
Previously, we passed the shared context to end_frame, now we pass the
actual GL context that the application uses for rendering. This is so
that the vfuncs could prepare the actual contexts for rendering (they
don't currently).

4. Simplify the code
The previous code set up the final drawing method in begin_frame.
Instead, we now just ensure the clip area is something we can render
and decide on the actual method in end_frame.
This is both more robust (we can change the clip area in between if we
want to) and less code.