Previously only tfra entries were added for the first track in each moof.

The frag_info array used for tfra can also be used for writing
other kinds of fragment indexes, where it's more important to
include all tracks.

When the separate_moof option is enabled (as in ismv), we write
a separate moof for each track, so this doesn't make any difference
in that case.
Signed-off-by: Martin Storsjö <martin@martin.st>