There are no behavioral changes, only some copy-and-pastes.

Before, drawFillOver was next to drawCopyOver.

After, drawFillOver is next to drawFillSrc, which is more similar to
drawFillOver than drawCopyOver is.

Similarly, drawCopyOver is now next to drawCopySrc, etc.

R=r
CC=golang-dev
https://golang.org/cl/4695045