Add image processing support for AVIF

The encode/decode is implemented in a WebAssembly module built from a
small C wrapper around libavif. Bundled libraries (statically linked,
compiled with the WASI SDK):

* libavif v1.4.1 (container + codec glue)
* libaom v3.14.1 (AV1 encoder + decoder)
* dav1d 1.5.3 (AV1 decoder)
* libyuv (Chromium pin) for color conversion
* parson for JSON message passing across the wasm boundary

HDR handling on the encoder:

* SDR images are written as BT.709 / sRGB / BT.601 (8-bit).
* 10-bit and up are written as BT.2020 primaries with PQ (SMPTE
  ST 2084) transfer and BT.2020-NCL matrix coefficients, signalled
  via CICP.
* Adobe-style SDR+gainmap inputs (e.g. Lightroom HDR exports) are
  baked into a single true-HDR image in BT.2020/PQ at 10-bit, with
  the CLLI (Content Light Level Information) box carried through so
  HDR-capable clients can tone-map correctly.

Limitations:

* Animated input (animated WebP/GIF) is collapsed to its first frame
  when re-encoded as AVIF; animated AVIF output is not yet supported.

Fixes #7837

go.sum

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