vout_mmal: Add MMAL-based vout module (WIP)

This module uses the MMAL api available on the Raspberry Pi. It is
still work in progress!

It features proper support for external resolution changes through the
TV service api. Subpicture support will be added later with DispManX
overlays.

Most importantly, it will feature zero-copy playback once the
MMAL-based decoder is finished and opaque pointers are used for the
pictures.