This splits out most of the OpenGL specific code from cogl-framebuffer.c
into cogl-framebuffer-gl.c and extends the CoglDriverVtable interface
for cogl-framebuffer.c to use.
There are hopes to support several different backends for Cogl
eventually to hopefully get us closer to the metal so this makes some
progress in organizing which parts of Cogl are OpenGL specific so these
parts can potentially be switched out later.
The only remaining use of OpenGL still in cogl-framebuffer.c is to
handle cogl_framebuffer_read_pixels.
