Add anisotropic filtering to GrSamplerState and implement in backends.

Currently unused.

Bug: skia:13036
Change-Id: I6f50a9218dbcd6c44b2c5745d6be6dbc0b00c06f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/527277
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
22 files changed