qtdemux: Add a minimum buffer size for raw audio to not output one buffer per frame

qtdemux: Add a minimum buffer size for raw audio to not output one buffer per frame

Outputting 48000 buffers per second is not a good idea performance-wise.
If a container sample is less than 1024 raw audio frames, combine
multiple samples to get at least 1024 raw audio samples as long as
they're stored contiguous in the file.

For the other direction, if a container sample contains more than 4096
samples there is already code for splitting them up.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=692750