Add directory portion when #including libavcodec headers.

This is how #including these headers is supposed to be done.
Replace deprecated uses of avcodec_get_chroma_sub_sample().

It has been superseded by av_pix_fmt_get_chroma_sub_sample().
Drop compiler warning flags workaround for obsolete libavcodec libraries.
Drop fallback struct and function definitions for obsolete libavcodec libraries.
Make exit state of a subprocess be fatal for the test
xvfb doesn't support some of the options we need, so stop trying to use it.

Need to find a better way to run dvswitch on a headless machine, I suppose...