Introduce MetaContextMain

This object intends to replace the scattered functions that are used to
make up what is effectively a "mutter context". It takes care of the
command line arguments that is now done in main.c, persistant virtual
monitors, and the like.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/core/meta-context.c

@@ -86,7 +86,7 @@ meta_context_notify_ready (MetaContext *context)
   META_CONTEXT_GET_CLASS (context)->notify_ready (context);
 }
 
-static MetaCompositorType
+MetaCompositorType
 meta_context_get_compositor_type (MetaContext *context)
 {
   return META_CONTEXT_GET_CLASS (context)->get_compositor_type (context);