[subtexture] Stop using cogl_material_unref
[subtexture] Stop using cogl_material_unref

This function is deprecated, replace it with cogl_handle_unref.

pwithnall/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx