video_output: add a CVPixelBuffer opengl provider
Note: this is currently a draft because it depends on !610 (merged) to be able to test it on MacOSX and !606 (merged) to avoid creating new leaks. I also want to double-test on iOS after the latest changes I made for MacOSX.
This offscreen OpenGL implementation is available for all Darwin platforms, either through EAGL for iOS/tvOS or through CGL for MacOSX.
Edited by Alexandre Janniaux