jhbuild: Add mesa-demos.
jhbuild: Add mesa-demos.

mesa has been split into the core libGL implementation and a demo
repository, so add a mesa-demos package.

freedesktop-unofficial-mirror/xorg__util__modular,freedesktop-unofficial-mirror/xorg__util__modular,tmzt/androix-util-modular,tmzt/androix-util-modular,freedesktop-unofficial-mirror/xorg__util__modular