cmdutils: Allow calling filter_codec_opts without a set encoder

In this case, no encoder specific options are filtered, only
options specific to that codec type in general.

Signed-off-by: Martin Storsjö <martin@martin.st>
avconv: Apply codec options to streams that are copied as well

This allows setting/overriding e.g. the bitrate parameter, which
is required for the smoothstreaming muxer. Normally, the bitrate
is set by the demuxer in these cases, but not all demuxers can
provide it. This allows stream copy of data to the smoothstreaming
muxer from such inputs.

Signed-off-by: Martin Storsjö <martin@martin.st>