Tl noninteger fps

Dear Martin,

Here a couple of commits related to issues with integer computations. I noticed some problems with a 23.97 fps video, that was identified as a 23 fps one, and the strip was off-sync... The main issues was in the PhVideoEngine::framePerSecond() method..

I have also fixed a potential problem with screen refresh rates that I am encountering here.

Thanks for your attention.

Best regards,

Timothée