converted GrRectBlurEffect to SkSL

Bug: skia:
Change-Id: I3a8e16fd2792e6fb5711815d8aad46ae30c2872e
Reviewed-on: https://skia-review.googlesource.com/59163
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
20 files changed