At this point only the gl driver is at all aware of the difference
between core and compat contexts. COGL_PRIVATE_FEATURE_GL_FIXED is also
now quite misnamed, since we're using the GLSL pipeline even for pre-GL3
contexts. Remove the private feature and handle the few remaining
differences by checking the driver class inside the gl driver.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/973