video/avcodec: do not use uninitialized AVFrame on stack

But use avcodec_alloc_frame()/avcodec_free_frame()
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=699722 for reference.
videostream: add pixconv filter iff encoder doesn't support source pixmt
v4l2: rework pixelformat selection

Prefer native over compressed over emulated as long as FPS for requested
resolution is >= 15 images/sec.