Rect types.
The actual image representation is unchanged. A future change will
replace the {[][]color} with {[]color, stride int, r Rectangle} and
possibly a clip region.
The draw.Color, draw.Point and draw.Rect types will be removed in a
future change. Trying to do it in this one polluted the diff with
trivia.
R=r, rsc
CC=golang-dev
https://golang.org/cl/1918047
Time to draw.Draw a 200x200 image fell from 18.4ms (and 1 malloc) to
5.6ms (and 0 mallocs). It's still relatively slow since it assumes
nothing about the src or mask images, but it does remove the malloc.
There are existing faster, more specialized paths for copies, fills
and image glyph masks.
Also added a "compare to a slow but obviously correct implementation"
check to draw_test.go.
R=rsc, r
CC=golang-dev
https://golang.org/cl/1223044
