These HLLs play .mpg movie files (later games use .alm file extension,
but the format is the same, i.e. MPEG-PS) that contain MPEG1 video and
MP2 audio streams.
pl_mpeg.h is the PL_MPEG library (https://github.com/phoboslab/pl_mpeg)
imported from commit 5aeef4d07593be1960a0e4c7fe204809cfae30bd.
This adds a few mixer functions (mixer_stream_*) for playing custom
audio streams under the master mixer.
