qtmux: Allow configuring the interleave size in bytes/time

qtmux: Allow configuring the interleave size in bytes/time

Previously we were switching from one chunk to another on every single
buffer. This wastes some space in the headers and, depending on the
software, might depend in more reads (e.g. if the software is reading
multiple samples in one go if they're in the same chunk).

The ProRes guidelines suggest an interleave of 0.5s is common, but
specifies that for ProRes at most 2MB (for SD) and 4MB (for HD) should
be used per chunk. This will be handled in a follow-up commit.

https://bugzilla.gnome.org/show_bug.cgi?id=773217