Add nearest MIP filtering to GPU and wire up to SkImageShader

Change-Id: I04b949f8962d35aed5c5e8b4d4290dc5157073d2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/305404
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
9 files changed