VideoReferenceClock: refactor, fixes, video sync for RPi

The old VideoReferenceClock has become a big mess and combined all platform specific code into a single file. So any fix or addition required a refactoring first. I stated with it because I wanted to get rid of the brittle GLX implementation for Intel systems. The invisible window with its GLX context often caused trouble.

In parallel @popcornmix worked on a clock for the Pi which is now included here.

credits to @Memphiz who did the work for OSX/iOS. In order not to break future bisects I squashed your changes into the refactor commit.