demux_lavf: use lavf for RealMedia (.rm) files by default

demux_lavf: use lavf for RealMedia (.rm) files by default

RealMedia was listed as a format for which the internal demuxer
(demux_real) was preferred over lavf. The original reason for this
(lavf failing to give any timing information for some video frames)
has been fixed in libavformat since. Make demux_lavf the preferred
demuxer for RealMedia.

The libavformat demuxer does still have issues. COOK audio initially
misbehaves after a seek (inconsistent timestamps, audio remaining from
the before-seek position). However, the internal demuxer seemed to be
_consistently_ out of sync with a test file. I haven't done thorough
testing, but the internal demuxer does not seem less buggy.