make all gather_*() use SkJumper_GatherCtx

SkJumper_GatherCtx is a prefix of SkImageShaderContext, so
this is a no-op.  It helps to keep things straight, and I
do want to split apart the GatherCtx from a new SamplingCtx.

Change-Id: I9c5f436b096624c2809e1f810e9bcd6c6b00b883
Reviewed-on: https://skia-review.googlesource.com/13264
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed