demux_lavf: set rawvideo codec_tag based on pix_fmt
Libav stopped automatically filling missing codec_tag field for raw
codecs based on pix_fmt in libav commit
bb416bd68c ("lavf: do not set
codec_tag for rawvideo"). This broke demux_lavf for raw video in
formats like YUV4MPEG, as the video format was not exported from
demux_lavf in any form (the information only existed in the pix_fmt
field of the struct AVCodecContext from libavformat, and that is not
exported). Add an explicit call to avcodec_pix_fmt_to_codec_tag() to
set the codec tag again so that selecting the correct raw decoder
based on the tag works.