Add a new BUFFER_AGE winsys feature and a get_buffer_age method to
cogl-onscreen that allows to query the value.
https://bugzilla.gnome.org/show_bug.cgi?id=669122
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Reviewed-by: Robert Bragg <robert@linux.intel.com>
Note: When landing the patch I made some gtk-doc updates and changed
_get_buffer_age to return an age of 0 always if the age feature isn't
support instead of using _COGL_RETURN_VAL_IF_FAIL. -- Robert Bragg
(cherry picked from commit 427b1038051e9b53a071d8c229b363b075bb1dc0)
860fb00fdc