Some features that were previously available as an extension in GLES2
are now in core in GLES3 so we should be able to specify that with the
gles_availability mask of COGL_EXT_BEGIN so that GL implementations
advertising GLES3 don't have to additionally advertise the extension
for us to take advantage of it.
Reviewed-by: Robert Bragg <robert.bragg@intel.com>
(cherry picked from commit 4d892cd97558da61ba526f947ac0555ebab632d2)
eb7c37812a