Update SkImageFilter's cache to handle SkSpecialImages and add unit test

This is calved off of: https://codereview.chromium.org/1695823002/ (Get OffsetImageFilter really working with SkSpecialImages)

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1709263002
TBR=bsalomon@google.com

Review URL: https://codereview.chromium.org/1709263002
6 files changed