Make GrRectBlurEffect use bilerp

Bug: skia:7031
Change-Id: I8061f516741bf7efd5020984a5b45a9d57307d43
Reviewed-on: https://skia-review.googlesource.com/62461
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed