qtmux: Fix ctts generation for streams that don't start at 0 timestamps

qtmux: Fix ctts generation for streams that don't start at 0 timestamps

Subtract the first timestamp of a stream from all input buffers to
get 0-based timestamps for creating a sane ctts table. Without this
patch the ctts could have larger values than needed, causing the
playback to have a delay at startup.

As the first timestamp is only found after a few buffers are queued
(due to possible reordered buffers), once we find the first timestamp
we subtract it from all buffers on the queue, from that point on,
all buffers have their timestamps subtract when they are collected.

https://bugzilla.gnome.org/show_bug.cgi?id=658659