ismindex: Verify that all bitrate variants match

In Smooth Streaming, the fragments are addressed by time, and
the manifest only stores one list of time offests for all streams,
so all streams need to have identical fragment offsets. Warn if
this isn't the case, so that the user can fix the files instead of
getting failures at runtime when the fragments can't be found.

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