vulkan: add an overlay layer

vulkan: add an overlay layer

Just a starting point to display frame timings & drawcalls/submissions
per frame.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
+1-by: Mike Lothian <mike@fireburn.co.uk>
+1-by: Tapani Pälli <tapani.palli@intel.com>
+1-by: Eric Engestrom <eric.engestrom@intel.com>
+1-by: Yurii Kolesnykov <root@yurikoles.com>
+1-by: myfreeweb <greg@unrelenting.technology>
+1-by: Kenneth Graunke <kenneth@whitecape.org>