obs-ffmpeg: Add QVBR rate control for VA-API

[nowrep]

Description

Add QVBR rate control for VA-API

Motivation and Context

This rate control is supported by both Intel and AMD, so make it available in OBS.

How Has This Been Tested?

Tested with Mesa (https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30221).

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.

[kkartaltepe]

Is there any reason to not use QVBR when its available instead of VBR?

[nowrep]

QVBR works differently than VBR, it's not "higher quality VBR".