Swap SkGpuBlurUtils over to using SkIRects

We don't have to land this, but I found it more comforting for the blurring code to explicitly deal with SkIRects rather than SkRects with integer values.

Split out of: https://codereview.chromium.org/1959493002/ (Retract GrRenderTarget from SkGpuBlurUtils)

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1968603003

Review-Url: https://codereview.chromium.org/1968603003
8 files changed