Simplify common case of linear filtering with no mips

Change-Id: I08b4a7bc088705b93a0aa680f6733d09c7ad23dd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/354221
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
31 files changed