output: Make implementations inherit 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 MetaOutput an abstract derivable type, and make the
implementations inherit it.

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

src/backends/native/meta-renderer-native.c

@@ -1436,7 +1436,8 @@ meta_onscreen_native_set_crtc_mode (CoglOnscreen              *onscreen,
     }
 
   meta_crtc_kms_set_mode (onscreen_native->crtc, kms_update);
-  meta_output_kms_set_underscan (onscreen_native->output, kms_update);
+  meta_output_kms_set_underscan (META_OUTPUT_KMS (onscreen_native->output),
+                                 kms_update);
 }
 
 static void