cogl: Port Buffer* away from CoglObject

Make CoglBuffer an abstract class and inherit the various Cogl*Buffer types from it.
As none of the subclasses is overriding the vtable functions, they were not turned into
vfuncs but plain function pointers in CoglBuffer.
We still use _cogl_buffer_initialize until we port the various params into actual construct-only
properties, similar to the previous commit for CoglTexture.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3193>

clutter/clutter/clutter-deform-effect.c

@@ -337,7 +337,7 @@ clutter_deform_effect_free_arrays (ClutterDeformEffect *self)
 {
   ClutterDeformEffectPrivate *priv = self->priv;
 
-  cogl_clear_object (&priv->buffer);
+  g_clear_object (&priv->buffer);
   cogl_clear_object (&priv->primitive);
   cogl_clear_object (&priv->lines_primitive);
 }