Fold clamp_{x,y} into the gathers.

All three image tile modes go through exclusive_clamp() and then a
gather today, so we can move the work of exclusive_clamp() into eac
gather_ stage, eliminating the need for clamp_{x,y} stages.

Luckily, we've got a convenient place to bottleneck this, ptr_and_ix(),
which works out the pointer and vector of indices to load for gathers.

This deletes SkRasterPipeline_repeat_tiling unit test, which now
no longer exactly makes sense.  It tests that repeat_x does that
clamp, but that's now done automatically outside that stage.

Change-Id: I24637ef60921bec7aa00082984c0c6a49dd86ca9
Reviewed-on: https://skia-review.googlesource.com/50260
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
7 files changed