Babystep preparations for explicit framebuffer management (EFM)

From the first commit's message:

This is in preparation for a future where only explicit frambuffer APIs are available, i.e., cogl_get_draw_framebuffer() does not exist.

There is absolutely no functional changes in this patchset, only rearrangements so that various functions receive a CoglFramebuffer instead of using the draw framebuffer.

At first, it might look that there's a conflict of goals here, since this branch ends up adding calls to cogl_get_draw_framebuffer() instead of removing them. This is not wrong, but it misses the point of this MR: prepare everything below Clutter layer (i.e. ClutterActor.paint() and ClutterActor.pick()) for receiving a framebuffer - via a new ClutterPaintContext that will be added - while drawing.