Add pass-through sampling to SkVM, use it when possible

This produces better shaders when using the arithmetic mode filter, for
example.

Change-Id: Ie1e030604c3a9c99122d112603a548b6fe359621
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300209
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed