v4l2-input: Support VAAPI decoding for v4l2 (MJPEG/H264)

[viric]

Description

Use VAAPI MJPEG/H264 decoding if available, using v4l2 cameras in Linux.

Motivation and Context

MJPEG decoding was using most of the OBS cpu, when I have two camera inputs in V4L2.

How Has This Been Tested?

I tested it in NixOS x86_64, with the patch over 30.1.2, which is what I had at hand. Then I also tested it on recent master, at exact commit hash provided.

Types of changes

• Performance enhancement (non-breaking change which improves efficiency)

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.