Added option --force-fps

[dronus]

A new option --force-fps creates synthetic timestamps to force playback at the given rate, overriding original media timestamps.

[popcornmix]

Can you explain what the use case is for this?

[dronus]

This allows for playing videos with a varying framerate at fixed speed (eg. screen refresh rate for example), play videos sporting bad timestamps with a fixed rate as fallback and so on.
For example I had one case where the RaspberryPi's raspivid tool stores DTS or PTS according to 25fps always even if not running at this rate. Those videos can then be played at different, desired speeds by omxplayer.

[dronus]

I also just wondered why omxplayer's --fps option wasn't working in many cases until I found out that it only applies to streams sporting no timestamps by itself.

[wallneradam]

Thank you very much for this PR! I have a raw H264 stream which don't have valid timestamp. Without this patch it plays as fast as it can, but now it is smooth.
It should be the default behaviour of "--fps" in case of wrong timestamps.