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...

[Noflare]

UP

[masc4ii]

@Noflare : No hardware to test here. I just see invalid options on my hardware which will bring the software to fail. If it works for you using the specified hardware, you could fork, build and use it as is.