optimize imageshaders to use clamp for dimension==1

Bug: skia:
Change-Id: I12021acb5349bd139dceee8ce46de3cfd1ff4d28
Reviewed-on: https://skia-review.googlesource.com/20698
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed