Clean up prior backdrop-filter CL

This addresses reviewer comments from https://skia-review.googlesource.com/c/skia/+/239108 that were postponed to get the first CL landed ASAP.

Additionally, removes the now unnecessary applyCTMForBackdrop() function.

Bug: skia:9074
Change-Id: Ibc3ac4a4edd7e5546fa83145346869a68efc33f4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/239440
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
4 files changed