Add a LD_PRELOAD-able library to force Metacube output from FFmpeg.

Add a LD_PRELOAD-able library to force Metacube output from FFmpeg.

This hooks just the right amount of functions to add Metacube output
to arbitrary FFmpeg programs, but is obviously very brittle.
(Native FFmpeg support would be better, but a patch did not
go through when I tried a while back.) It is only lightly tested.
Documentation in the README and cubemap.config.sample.