image/draw: add draw tests where the destination image doesn't start at (0, 0).
image/draw: add draw tests where the destination image doesn't start
at (0, 0).

Also refactor the test to use the SubImage method rather than monkeying
with an image's Pix and Rect fields.

R=r
CC=golang-dev
http://codereview.appspot.com/4678045
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go