image: roll back 13239051 (add RGB and RGB48)

image: roll back 13239051 (add RGB and RGB48)

They cause too much bloat in the internals as we find ourselves adding
special case code for all the cross-connections. It's better to use RGBA
and just max out the alpha. We lose a little memory but reduce the number
of special cases the encoders, decoders, and drawers need to provide.

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/42910045