color-manager: Add API to get the color device for a monitor

There is a 1-on-1 mapping between these two entities, so add a getter.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2164>

src/backends/meta-backend-types.h

@@ -23,6 +23,7 @@
 
 typedef struct _MetaBackend MetaBackend;
 
+typedef struct _MetaColorDevice MetaColorDevice;
 typedef struct _MetaColorManager MetaColorManager;
 
 typedef struct _MetaMonitorManager MetaMonitorManager;