draw: implement Kernel.Transform.

Also fix the NN and ABL fast paths to only apply if we can access the
Pix elements without src-bounds checking.

Change-Id: Ie9fc96b28e0665df49d00c4c53cb81385faee4db
Reviewed-on: https://go-review.googlesource.com/7675
Reviewed-by: Rob Pike <r@golang.org>
6 files changed