[img-decode] Start on proposed new spec

This adds readPixel and a partial stub of window.createImageData

Change-Id: Iee992312b9331b71852fe2198f844a7e4ae9e963
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/211344
Reviewed-by: Mike Reed <reed@google.com>
8 files changed