This deprecates clutter_stage_read_pixels since there are numerous
issues with that interface, including lacking of format choice and
difficult to support semantics considering read_pixel requests that
happen mid-scene.

As a replacement this adds clutter_stage_read_pixels_async which serves
a similar purpose but instead of synchronously painting the stage it
queues a clipped redraw of the region of interest and after the stage is
next painted the region is read and a user given callback is called to
notify that their pixel data is ready.

The new API allows control over the pixel format that pixels are
returned in, there is the potential for us to optimize this api to use
PBOs in the future to completely avoid blocking the mainloop while
waiting for the reads to complete, we avoid redundant scenegraph
traversals and avoid tricky semantics when reading mid-scene.