Add formats, frameserver and accompanining helper code
Main part of this MR is the frameserver interface and accompanying v4l2 driver. The rest is helper code for dealing with formats and converting frames. This is a much smaller interface then what @pblack did on the montrack branch. It's in no way final and was reduced so it would be easier to change or add to the interface.
Currently not hooked up in the prober but the code has been tested with a experimental SDL2 based gui that currently lives on the jakob-montrack-to-xrt
branch.