Split up find_or_create_rrect_blur_mask_fp

Prepare this method for cpu-side generation of the rrect blur mask in DDL-mode

Bug: 1108408

Change-Id: I902ad507cd53f5f5b1d418a8b52f96dddffd0b18
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/314621
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed