mov: Allow more than one keyframe per trun

mov: Allow more than one keyframe per trun

The previous restriction was partially designed to fix certain
(broken) samples from bug 215. There should be no restriction on the
number of keyframes per fragment or trun.

The spec suggests that all frames lacking MOV_FRAG_SAMPLE_FLAG_IS_NON_SYNC
are key frames, but we require the flag MOV_FRAG_SAMPLE_FLAG_DEPENDS_YES
to be unset as well. This works for (possibly broken) media that never
sets the NON_SYNC flag and should also be correct for any spec-compliant
file.

For files that never set either of the flags, all samples are marked
as keyframes.

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