MP4 Encoding support

[torre76]

The idea is to replicate MKV encoding that supports MP4. Process is:

• Reencode video thru mencoder
• Extract audio thru MP4Box after gaining information with mp4info (which should be included into exec requirements)
• Remux MP4 into final folder (without sub files inside for the moment) using MP4Box

[gquadro]

See commit adf6df3e3d78523c848a2bc2f257953a33ce2f25

The files with audio tracks need to have extensions corresponding to their type for MP4Box to work. At the moment the only known type is AAC for lack of MP4 files.

In _build_mp4_finalfile() the subbed but mute video generated by mencoder is extracted through MP4Box for when muxing it requires a file that starts with a keyframe.

[torre76]

To fix _build_mp4_finalfile() issue on keyframe it is possible to encode a raw video x264 stream (without any container) that MP4Box handles correctly, thus the container extraction step would be avoided.

mencoder needs this extra param to do this:

-of rawvideo

Also the filename should ends with .264 (to avoid MP4Box problem with file detection).

[gquadro]

See commit 63fe4de07478d7153aa939fca68d4ad0fff85ea9