Support toBuffer(cb, mimeType, quality)?

[zbjornson]

Some discussion in #743, but in essence there should be a non-streaming API for all formats with all config options (e.g. quality, PNG filter).

toBuffer(cb: function, mimeType?: string, quality?: number) would parallel toBlob.

toBuffer(cb, "image/jpeg", quality: number);
toBuffer(cb, "image/png", compressionLevel: number);
// accommodate non-standard options:
toBuffer(cb, "image/png", {compressionLevel?: number, filter?: number, palette?: Uint8ClampedArray, backgroundIndex?: number});
toBuffer(cb, "image/svg"); // throws if not an SVG canvas
toBuffer(cb, "raw"); // Maybe? No need for this to use a callback as it's just a memcpy.
toBuffer(cb, "image/gif"); // maybe in the future

Alternatively, @adamhooper proposed separate toPNG/toJPEG functions.

Should there be sync versions that return a value instead of accepting a callback?

See also #934 about cleaning up the PNG filter API.

I can work on this sometime soon. Feedback welcome.

[jimmywarting]

Can we depend on fetch-blob to make canvas.toBlob function in nodejs too?