Rearrange NN sampling to more naturally support bilerp.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5105

Change-Id: Ic692b5faf2d33fee31b119ff8d3653118b25b7c2
Reviewed-on: https://skia-review.googlesource.com/5105
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed