Use faster dav1d codec for decoding avif files

[jamsinclair]

Fixes #43

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[jamsinclair]

For anyone viewing this branch there is an issue with returning the RGB image data to the JavaScript context from Wasm.

libavif seems to compile fine with dav1d. I have also tested the decoded pixel values look correct.

However, when I try and pass the pixel data back to Javascript I encounter some kind of memory issue

avif_dec.js:8 Uncaught (in promise) TypeError: caller is not a function
    at __emval_call (avif_dec.js:8:27103)
    at avif_dec.wasm:0x7c15d
    at avif_dec.wasm:0x7c39d
    at Object.decode (avif_dec.js:8:13828)
    at Module.decode (decode.js:24:27)
    at async decode (main.js:10:14)
    at async convert (main.js:42:21)
    at async HTMLFormElement.<anonymous> (main.js:74:25)

I've traced this back to this part the code

jSquash/packages/avif/codec/dec/avif_dec.cpp

Lines 30 to 34 in 201aa03

	// We want to create a *copy* of the decoded data to be owned by the JavaScript side.
	// For that, we perform `new Uint8Array(wasmMemBuffer, wasmPtr, wasmSize).slice()`:
	result = ImageData.new_(
	Uint8ClampedArray.new_(typed_memory_view(rgb.rowBytes * rgb.height, rgb.pixels)), rgb.width,
	rgb.height);

I've spent too much time on this for now, so I'll let it be. There's no urgency.