tweaks to make gather_* easier in SkJumper

This moves all the values that gather_8888, gather_a8, etc. need to
the front of SkImageShaderContext, and dereferences the color table.

This should be a no-op, but will make these stages easier to write
in SkJumper.

Change-Id: I0dff97d5113d14e941e7b717cd85f0036764eb88
Reviewed-on: https://skia-review.googlesource.com/11492
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed