fix repeat/mirror sampling bleed

I think this has been broken since we tried to simplify this in
  https://skia-review.googlesource.com/16547

The HSW backend does still look a little wrong, but improved,
and the others seem fixed.  Can you see how this affects your
test cases, layout tests, etc?

BUG=skia:6783

Change-Id: I17957ac8100331bea5b64d674bf43105048b72f6
Reviewed-on: https://skia-review.googlesource.com/20548
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
6 files changed