Rewrite blank_background

New blank_background version permit to introduce mute video
https://github.com/popcornmix/omxplayer/issues/66

https://github.com/fcarlier/omxplayer/commit/d89ea34e937e9e0c94fcba3b2e2d5927004ca4de
https://github.com/fcarlier/omxplayer/commit/667f8d7d7c69607f06ca672c8c000dd474435204