There are several containers that support adpcm_thp (Also known as Gamecube DSP)
streams, but only thp files contain the coeff table and previous sample inside
each frame.
Some don't even contain previous sample information at all.

This change will make it easier to implement demuxers for said containers
without having to create a new decoder.
Signed-off-by: James Almer <jamrial@gmail.com>