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

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

(cherry picked from commit 4fdb8b966ad3501842bbdff3d96b0c4e098e2646)

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