feat: VA-API support for linux. (hw accelerated video)

[cloudwebrtc]

vaapi works now, but there are some things need to confirm.

• can call drm driver for h264 encoding for AMD/APU
• check rate control is effective
• confirm encoder adapts to resolution changes
• check support for Intel CPU
• check and confirm for H.264 encoding in baseline, main & high profiles
• simulcast support (maybe in next commit)
• add capability detection support for vaapi encoder factory

[cloudwebrtc]

hi @iliabaranov, just compiled liblivekit_ffi.so with vaapi support in GH Actions. do we have any related programs to test it?
https://github.com/livekit/rust-sdks/actions/runs/16794133822

[iliabaranov]

For sure!