Averaged times (in microseconds) for drawing an 800x600 rectangle
are listed below. The summary is: around a 100x improvement.

draw.Draw call times were typically linear in the number of pixels
touched (i.e. drawing an 800x600 rectangle took 100x as much time as
drawing an 80x60 rectangle).

Before this change, there was only the general-but-slow code path.
When drawing any src with a 50%-opaque mask: 237300 us
When drawing any src with a nil mask: 50100 us

After this change, the 50%-opaque mask case is unchanged.
For an *image.RGBA dst and nil mask and...
...a uniform color (i.e. an image.ColorImage) src: 282 us
...another *image.RGBA src: 615 us.

For the curious, an intermediate implementation detected the special
cases but used simple nested for loops instead of the built-in copy
function. The respective times (compared to 282 and 615 for the
final implementation, or 50100 for the original) were 3110 and 3573.

Times were measured with 8g/8l on my laptop. I haven't tried gccgo
or other architectures.

R=r, rsc
CC=golang-dev
https://golang.org/cl/201048