Add two optional filter to video src

Add two optional filter to video src

Add videomaxrate and postproc_tmpnoise as optional filters to
empathy_video_src_init. Thay will used if thay are installed.

videomaxrate will set maximal framerate to 15fps if some haw webcam
will get more than 15fps it will not kill empathy-av with high CPU load.

postproc_tmpnoise will reduce video noise produced by most (all) webcams.
This will add smoll overhead for load but reduce CPU load of video encoder.
Also this should reduce network load (for theoraenc x2)

Signed-off-by: Alexey Fisher <bug-track@fisher-privat.net>