gl_common: remove unneeded callback

We got rid of this some time ago, but apparently not completely.
vd_lavc: select correct hw decoder profile for constrained baseline h264

The existing code tried to remove the "extra" profile flags for h264.

FF_PROFILE_H264_INTRA doesn't matter for us at all, because it's set
only for profiles the vdpau/vaapi APIs don't support.

The FF_PROFILE_H264_CONSTRAINED flag on the other hand is added to
H264_BASELINE, except that it makes the file a real subset of H264_MAIN
and H264_HIGH. Removing that flag would select the BASELINE profile,
which appears to be rarely supported by hardware decoders. This means we
accidentally rejected perfectly hardware decodable files. Use MAIN for
it instead.

(vaapi has explicit support for CONSTRAINED_BASELINE, but it seems to be
a new thing, and is not reported as supported where I tried. So don't
bother to check it, and do the same as on vdpau.)

See github issue #204.
gl_common: print SW renderer warning only if it was the only reason we rejected it
audio: fix audio data memory leak

Practically all audio decoding and filtering code leaked sample data
memory after uninitialization due to a simple logic bug (or typo).
demux: use talloc for certain stream headers

Slightly simplifies memory management. This might make adding a demuxer
cache wrapper easier at a later point, because you can just copy the
complete stream header, without worrying that the wrapper will free the
individual stream header fields.