For video, mark the first sample in a trun which doesn't have the
sample-is-non-sync-sample flag set as a keyframe.

In particular, the "sample does not depend on other samples" flag
isn't enough to make it a keyframe, since later frames still can
reference frames prior to that one (the flag only says that that
particular frame doesn't depend on other frames).

This fixes bug 215.
Signed-off-by: Martin Storsjö <martin@martin.st>