Add a makeDrawContextWithFallback that handles config fallback

This fixes a bug in find_or_create_rrect_blur_mask where an A8-based drawContext was desired but creation was failing b.c. A8 wasn't renderable.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2296193005

Review-Url: https://codereview.chromium.org/2296193005
9 files changed