Make SkGpuBlurUtils::GaussianBlur more drawContext centric

This is 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=1962903003

Committed: https://skia.googlesource.com/skia/+/e7ef01dcfda0f8ae407ba92cc03cf3f7841ec470

Review-Url: https://codereview.chromium.org/1962903003
4 files changed