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>
8 files changed