Add an option to export H.264 with VAAPI GPU acceleration

[grego]

This enables exporting H.264 with GPU acceleration on all Intel and AMD GPUs from the past decade.

[masc4ii]

Thank you very much for your pull request. And please excuse the very late response - big lack of time atm here.
I downloaded and tested your code on my MacBookAir M1 - sure, it can't work here. But the option is visible, while other invalid options are hidden (like AVFoundation on non-macOS). I'll have to search a non ARM computer for more tests...