Screen for 32-Bit Octave on Linux rebuilt and tested. Fix 16 bpc dual-display.

-> Formatting shader needed minor fix to enable proper 16 bpc
   framebuffer mode on single-display, dual-display mirror mode
   and also dual-display side-by-side "stereo" mode.

-> Screen.mex rebuilt and tested. Works nicely.
Add xorg.conf file for preparing AMD gpu's for 16 bpc scanout.

Psychtoolbox/PsychGLImageProcessing/xorg.conf_For_AMD16bpcFramebuffer
is a minimal xorg.conf file to prepare AMD gpu's for our 16 bpc
framebuffer hack.

It switches scanout and rendering to non-tiled linear framebuffer,
so the framebuffer format is what we actually expect for our dirty
hack to work.