This reverts commit 7feef7db.

This commit causes assertion failures due to use of parser_init from multiple
threads, for example indirectly by ffmpeg.c and more directly from the
packet read functions.

I dont know how to fix this quickly, and fixing ffmpeg.c leaves
the possibility of other applications being affected.
Crashing the applications until this is resolved is clearly no good
thus this revert, so we have time to think about the problem.

Crashes can be reproduced by using multiple input files in ffmpeg.