Add new GM for occluded blurmask filter draws

This splits out just the GM portion of https://codereview.chromium.org/2201993003/ (Add GM to test out blurmaskfilter occluders) which included some API changes

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2214033002

Review-Url: https://codereview.chromium.org/2214033002
3 files changed