image: add AtRGBA, AtGray16, etc. methods symmetric with SetRGBA, SetGray16, etc.

[nigeltao]

The image.Image and draw.Image interfaces define At and Set in terms of color.Color, an
interface type.

The concrete image implementations image.RGBA, image.Gray16, etc. also define SetRGBA,
SetGray16 etc. for performance compared to a vanilla Set.

We should consider also providing AtRGBA, AtGray16, etc.

[gopherbot]

Comment 1:

CL https://golang.org/cl/109000049 mentions this issue.

[nigeltao]

Comment 2:

This issue was closed by revision ca94064.

Status changed to Fixed.