Fix the issue of "Tag <blah> incompatible with output codec id '<blah>'"

This little fix solves the above problem where libavformat refuses to write the header while complaining about an obscure tag value.

It turns out that libavformat doesn't like the codec_tag that is copied into the output stream codec context from the input stream codec context. Setting this to 0 allows the correct codec_tag for the output container format to be selected when the header is written.