cogl-gles2-context: use new _add_frame_callback api
cogl-gles2-context: use new _add_frame_callback api

This updates cogl-gles2-context to use the new
cogl_onscreen_add_frame_callback() api to use _SYNC events for
throttling.

(cherry picked from commit 2063306645e32ceb3252ebcd2eebd33c879d7fbe)

Distrotech/cogl,Distrotech/cogl,Distrotech/cogl,Distrotech/cogl