PsychImaging/Screen on Linux: Add experimental 16 bpc framebuffer support for AMD gpu's.

PsychImaging('AddTask','General','EnableNative16BitFramebuffer') allows to request
use of an up to 16 bpc / 48 bit color depth framebuffer. This is only supported on
Linux + X11 + FOSS graphics drivers for AMD gpu's, and implemented by a new series
of low-level gpu hacks, shaders and other naughtyness to switch the AMD gpu into
64 bpp framebuffer, RGBA16161616 scanout mode and format framebuffer content
accordingly. While this establishes a 16 bpc framebuffer, the display pipelines
of current AMD gpu's are not fully 16 bpc, so some truncation/loss of precision will
occur. "Sea Islands" family gpu's can do up to 12 bpc native output on DisplayPort
and HDMI in deep color modes, up to 10 bpc native + 2 bpc dithered over analog VGA
output. Older gpu's can only do up to 10 bpc, so there's no real use for this new
function on older than "Sea Island" gpus, as our older 10 bpc and 11 bpc native fb
support already covers the range up to 10 bpc perfectly at a lower complexity and
performance hit. This code still allows to drive those gpus, as i currently only
have access to pre-Sea Island gpus, so for testing this must do, and actual validation
of this approach via photometer etc. will be left as an exercise for some proud owner
of a latest generation AMD Sea Islands+ gpu.

This hack not only requires PTB magic in framebuffer setup and output formatting,
but also use of a special xorg.conf file to make sure the framebuffer and all rendering
is done in linear mode without any framebuffer tiling, as otherwise the low level data
formatting would go haywire.

The current code is partially prepared for dual-display operation, e.g., stereo,
but some bits of shader code and setup code is missing. First get it working on
single display.

-> Screen('Resolution', ...., specialflags & 2) now allows to only resize the
   X-Screen / its framebuffer, even on pure single display setups, as we need
   the ability to resize the framebuffer/X-Screen to twice the height - and sometimes
   width - of the viewport of the scanout for this trick to work.

-> Screen has proper low level mmio gpu scanout setup code for 32 bpp -> 64 bpp scanout
   switching on AMD gpus.

-> Screen for 64-Bit Octave on Linux rebuilt and tested, but not yet on an actual
   AMD gpu.