obs-ffmpeg: Fix hardware decode failed with av1 video

[Hlongyu]

Description

Add support for av1 hardware decode

Motivation and Context

#10213
https://github.com/mpv-player/mpv/blob/112fa549beb52ab3d75999c8d8a82936e13001c4/video/decode/vd_lavc.c#L312

How Has This Been Tested?

Tested on Windows11 nvidia 3060 laptop with 8k av1 video

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

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

[Hlongyu]

When decoding AV1 video, ffmpeg utilizes libdav1d as the optimal decoder, which does not support hardware decoding. The only decoder that supports hardware decoding is named "av1," but it does not support software decoding.

Therefore, if an attempt is made to decode an AV1 video using hardware, the "av1" decoder will be employed. If decoding fails due to unsupported GPU capabilities, OBS should switch to libdav1d for software decoding. However, it is uncertain whether OBS will make this switch. Could someone verify this?