mov.c: allow reading fragment start dts/pts from fragmented mp4

This introduces a new option to the mov demuxer: -use_mfra_for
(pts|dts). When it's given and moofs and a MFRA are present, the MFRA's
TFRAs are read for fragment start times.

Unfortunately some programs that produce fragmented mp4s use the TFRA
time field for dts and some for pts. There is no realistic way to detect
which is the case, hence the responsibility is punted onto the user.
This also means that no behavioural change is enabled by default - you
must pass either dts or pts for anything to happen.

Without this change, timestamps for some discontinuous fragmented mp4 are
wrong, and cause audio/video desync and are not usable for generating
HLS.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>