image: add AtRGBA, AtGray16, etc. methods symmetric with SetRGBA, SetGray16, etc. #7694
Description
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.