libva accelerated encoding

[Antoine Martin]

Split from #370, see also #202 for the decoding side.

The only hardware supported by libva for accelerated encoding are some Intel chips.

Pointers:

• ​libva
• ​qemu patch (good starting point / example)

Note: libva can encode to other formats (avc, mpeg2, ..) and maybe this can be useful in the future.

[Antoine Martin]

Raising, Intel's ​Valley Vista makes this much more interesting. Awaiting hardware to take this on.

[Antoine Martin]

stub implementation

[Antoine Martin]

Spotted: ​H.264 Encoding On Skylake Gets A Big Performance Boost, details here: ​Add Low power, High performance h264 encoder support.

[Antoine Martin]

Could be used for 10-bit colour too, see #909.

[Antoine Martin]

As per ​Fedora 28 Planning For VA-API 1.0 Support, Libva 2.0 was released last October with H.264 FEI support in its API, deprecating older parts of the API, fixing a race condition with the Wayland support, renaming some parts of the API, improving the logging capabilities, and various other changes. Libva 2.0 broke API/ABI compatibility with older versions of this Intel-developed Video Acceleration API.

We want to target libva 2.0, so let's wait for Fedora 28 beta.

OBS does it through ffmpeg: ​Add linux VAAPI h.264 encoding support - I'm not sure we should do the same: ideally we can reduce the dependency on ffmpeg, not increase it.