clutter: Add infrastructure to render ClutterStageViews to offscreen

The offscreen is given through the ::back-buffer property, the ClutterStageView
will set up the the CoglPipeline used to render it back to the "onscreen"
framebuffer.

The pipeline can be altered through the setup_pipeline() vfunc, so ClutterStageView
implementations can alter the default behavior of blitting from offscreen to
onscreen with no transformations.

All getters of "the framebuffer" that were expecting to get an onscreen have
been updated to call the right clutter_stage_view_get_onscreen() function.

https://bugzilla.gnome.org/show_bug.cgi?id=745079