Enable native AV1 VideoToolbox decoding on macOS

[elibosley]

What changed

Enable FFmpeg's native AV1 decoder in the macOS dependency build.

The build already enabled av1_videotoolbox, but only built the libdav1d
decoder. FFmpeg exposes the VideoToolbox hardware configuration through its
native av1 decoder, so Moonlight could not discover AV1 hardware decoding and
fell back to HEVC when hardware decoding was required.

libdav1d remains enabled as the software fallback.

Fixes the dependency-side regression still reported in
moonlight-stream/moonlight-qt#1125.

Validation

• Built the pinned FFmpeg 8.1 commit on Apple Silicon.
• Verified configure reports:
  • decoders: av1, libdav1d
  • hwaccel: av1_videotoolbox
• Verified avcodec_get_hw_config() exposes videotoolbox_vld for the native
  AV1 decoder while libdav1d remains available.
• Built current Moonlight Qt master against the resulting dependency.
• Streamed AV1 Main10 HDR at 3024×1964, 60 FPS on an M4 Pro with hardware
  decoding forced. Moonlight negotiated AV1 Main10 (format 0x2000) and FFmpeg
  selected videotoolbox_vld successfully.

[cgutman]

I don't think this is actually necessary, since FFmpeg's build system automatically enables the av1 decoder since we're building the av1_videotoolbox hwaccel.

You can see it's being properly enabled in the FFmpeg configure log even without this patch:

Enabled decoders:
av1                     hevc
h264                    libdav1d

It doesn't hurt to be explicit though, so I'll merge this anyway.