converted GrMagnifierEffect to SkSL

Bug: skia:
Change-Id: I6dc14ac66d5b911117e71fa23fef49a897082781
Reviewed-on: https://skia-review.googlesource.com/71342
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
24 files changed