Remove texture sampling from GrConfigConversionEffect

Re-land fixed version of https://skia-review.googlesource.com/c/10026/

BUG=skia:

Change-Id: I0754ffb72da2966eb57e5cd7ec818b1cdce84a74
Reviewed-on: https://skia-review.googlesource.com/10056
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed