[graphite] Set up SkSL portion of image shader

Bug: skia:12701
Change-Id: I4d1fc66307a5c4f98f5415f95670b1ebc1622f97
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/523859
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed