[canvaskit] Clean up readPixels API on Canvas and Image

This makes both APIs have the same arguments with the two
source coordinates first and all the destination params
(image info, optional buffer, optional rowBytes) after.

Bug: skia:10717
Change-Id: I483e4f33f24e226793db6113d5ba5b1955cd892e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/332622
Reviewed-by: Mike Reed <reed@google.com>
7 files changed