This will also expose the Vulkan precompilation path to immutable samplers.
Change-Id: Ida31bd70455299fbcc8f4d728aa15179f7685311
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295799
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
