crtc: Make implementations inherit MetaCrtc

Just as with MetaOutput, instead of the home baked "inheritance" system,
using a gpointer and a GDestroyNotify function to keep the what
effectively is sub type details, make MetaCrtc an abstract derivable
type, and make the implementations inherit it.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1287

src/tests/monitor-test-utils.c

@@ -560,7 +560,7 @@ create_monitor_test_setup (MonitorTestCaseSetup *setup,
     {
       MetaCrtc *crtc;
 
-      crtc = g_object_new (META_TYPE_CRTC,
+      crtc = g_object_new (META_TYPE_CRTC_TEST,
                            "id", i + 1,
                            "gpu", test_get_gpu (),
                            NULL);