Revert "Revert "Add support for image load to SkSL"" This reverts commit cb115bdeed5898ded3fdbe572a14616cff809b7c. GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4900 Change-Id: Ibcb381bae83d0cfc1a1226be90792061d401426a Reviewed-on: https://skia-review.googlesource.com/4900 Reviewed-by: Brian Salomon <bsalomon@google.com> Commit-Queue: Brian Salomon <bsalomon@google.com>