Don't rely on primProc textures when adding samplers

Change-Id: Idee88e061004c308665ad5e7c56528398c8e7201
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/274842
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
12 files changed