If the demuxer did not set a codec_tag, there is none and
inventing one makes no sense.  This change stops the rawvideo
"decoder" over-writing user-supplied pixfmt with one derived
from the codec_tag.  The pixfmt-codec_tag-pixfmt round-trip
is lossy since several pixfmts map to the same codec_tag.

This fixes fate-lavf-pixfmt with avfilter disabled.
Signed-off-by: Mans Rullgard <mans@mansr.com>