Add ffmpeg vulkan support

[tormento]

There is vulkan implementation in some builds of ffmpeg.

Would be possible to decode video by hardware, thus accelerating the output?

[jonfryd]

Hi @tormento,

Thanks for sharing your idea!

Yes, it should be doable by lending some ideas and patterns from https://github.com/FFmpeg/FFmpeg/blob/master/doc/examples/hw_decode.c

I'm not going to work on this (there is a fair amount of work involved and I rarely need the extra performance boost), but I'm open to PRs.