cogl: deprecate cogl_draw_buffer API and replace with a cogl_framebuffer API

cogl_push_draw_buffer, cogl_set_draw_buffer and cogl_pop_draw_buffer are now
deprecated and new code should use the new cogl_framebuffer_* API instead.

Code that previously did:
    cogl_push_draw_buffer ();
    cogl_set_draw_buffer (COGL_OFFSCREEN_BUFFER, buffer);
    /* draw */
    cogl_pop_draw_buffer ();
should now be re-written as:
    cogl_push_framebuffer (buffer);
    /* draw */
    cogl_pop_framebuffer ();

As can be seen from the example above the rename has been used as an
opportunity to remove the redundant target argument from
cogl_set_draw_buffer; it now only takes one call to redirect to an offscreen
buffer, and finally the term framebuffer may be a bit more familiar to
anyone coming from an OpenGL background.

doc/reference/cogl/cogl-sections.txt

@@ -245,6 +245,9 @@ cogl_offscreen_new_to_texture
 cogl_offscreen_ref
 cogl_offscreen_unref
 cogl_is_offscreen
+cogl_set_framebuffer
+cogl_push_frameffer
+cogl_pop_framebuffer
 cogl_set_draw_buffer
 cogl_pop_draw_buffer
 cogl_push_draw_buffer