h.264: fix bitstream timing info: write proper values for 25000/1001 fps
Fix seek pre-roll and codec delay mapping.

The muxer was incorrectly writing the 'pre-skip' header from
the Ogg Opus header as the SeekPreRoll element instead of
the CodecDelay element, and vice-versa.
Enable the Opus codec in WebM.
AUTHORS and ChangeLog: Updates for e852ab4 and 3170ac7

e852ab4 Fix seek pre-roll and codec delay mapping.
3170ac7 Enable the Opus codec in WebM.
tests: update due to e852ab4 Fix seek pre-roll and codec delay mapping.
test framework: sys returns both the output and the exit code
tests: add test case for muxing various formats into WebM