Screen: Add new optional 'bitdepth' parameter to 'OpenVideocapture'

This will allow to request bpc bit depth values for video capture
other than the classic 8 bpc. Also update help text and remove some
dead code related to the non-dma fallback of libdc1394, which doesn't
exist anymore since years.

Now we just need to implement > 8 bpc bitdepth support in at least the
dc1394 backend, as a starter.
Screen: Add > 8 bpc capture support to libdc1394 video capture.

A new optional parameter 'bitdepth' in Screen('Openvideocapture')
allows to request video capture with more than 8 bpc color/intensity
resolution.

The parameter is currently silently ignored with the GStreamer capture
engine.

On the dc1394 engine, requesting > 8 bpc will cause the engine to
request a 16 bpc video format from the camera. This leads to failure
if the camera doesn't support > 8 bpc. If the camera supports > 8 bpc,
video capture of a 16 bpc stream is done and video data is returned
accordingly, either as Matlab/Octave matrix of uint16 format, or as
Psychtoolbox textures of 16 bpc or 32 bpc floating point format.

If the net payload is <= 10 bpc, then 16 bpc float textures are used.
Net payloads > 10 bpc are returned inside 32 bpc float textures to
preserve the full color resolution.

Add new function to scripting glue: PsychAllocOutUnsignedInt16MatArg()
to allow return of uint16 matrices.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
-> Tested successfully on a 12 bpc firewire camera in both luminance
   and color mode, (MONO 16 --> Bayer --> RGB16 in case of color mode).

-> Videocapture & texturing tested, test of return of uint16 data to Octave pending.