Reland "Switch runtime SkSL to always sample at explicit coords"

This reverts commit d4bf54eac6243b2b5c2125f6e696d880bbe3ec33.

Change-Id: I65bfea4d880de29394e25d44d781fd18508fe337
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266942
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed