image: change Pix[0] to mean top-left corner of an image's Rect instead

image: change Pix[0] to mean top-left corner of an image's Rect instead
of the origin.

image/png and image/jpeg benchmarks show no significant changes.

The image/draw changes suggest to me that making a gofix for this is not
feasible. People are just going to have to make manual fixes.

R=r
CC=golang-dev
https://golang.org/cl/4681044