[img-conversion] create function (#19)

Promise<ImageData>  create(HTMLImageElement for Blob).       Does we need some parameter to specify the output format? Is the RGBA8888 or RGB565?  Is the alpha channel per-multiplied or keep as an independent channel, etc 

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/img-conversion/issues/19