movdec: Don't parse all fragments if ignidx is set

movdec: Don't parse all fragments if ignidx is set

In nonseekable files, we already stop parsing the toplevel atoms
after finding moov and one mdat. In large seekable files (or files
that are seekable, but slowly, e.g. http), reading all the fragments
at the start can take a considerable amount of time. This allows
opting out from this behaviour.

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