Fix, cleanup and document the clamp tiler better.

* Fix by making the bounds where the bounds of the edge effect starts to be .5 pixels all around the border.
* Sample from the center of the pixel. This was wrong on the left edge.


GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5127

Change-Id: I39af942644fa58f0ee8dc7027481fef85b9488fe
Reviewed-on: https://skia-review.googlesource.com/5127
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
1 file changed