Add support for input attachments in SkSL spirv

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5115

Change-Id: I3e03a465a10c9aff62491d0f6e71105d1b650dab
Reviewed-on: https://skia-review.googlesource.com/5115
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
11 files changed