qtdemux: Support sidx based seek in pull mode

Submitted by Seungha Yang

Link to original bug (#764629)

Description

qtdemux: Support sidx based seek in pull mode

tfra atom assists seeking to random access point. Because it is not mandatory,
however, it is not desirable to rely only on it. In this context, sidx box
can help seek for fragmented file.

This patch enables qtdemux to seek for fragmented file if only sidx available
(i.e., there is no tfra box in a file).