build: Add a Meta Toolkit private library

Currently, Meta/Cogl/Clutter makes use of cairo_rectangle_int_t despite
the existance of MetaRectangle.

In order to make MetaRectangle usable in Cogl/Clutter as well, Mtk would
provide such base types that are shared across the various private
libraries

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3128>

doc/reference/mtk/urlmap.js

@@ -0,0 +1,7 @@
+// SPDX-FileCopyrightText: 2021 GNOME Foundation
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+// A map between namespaces and base URLs for their online documentation
+baseURLs = [
+    [ 'GObject', 'https://docs.gtk.org/gobject/' ],
+]