Add xorg.conf sample file for 30 bit, 10 bpc framebuffers.

This is a minimal xorg.conf file which would enable
native 10 bpc, 30 bit color depth support on gpu's
whose hardware + graphics driver support this mode
natively, ie., as builtin OpenGL feature, without
any need for PTB special hacks.

-> Would work on NVidia binary driver, and will work (as tested)
   once we integrate our patches into upstream Mesa and radeon-ddx,
   which is tbd.