Centralize GrSamplerState key creation

This also changes how the Metal backend computes its key (removing the # of mipMap levels) and removes the use of the # of mipMap levels when setting up the MTLSamplerDescriptor

Change-Id: I29fedc7c7a12773f3efeea824bca98cbcde98be8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/248565
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed