Switch SkMagnifierImageFilter over to new onFilterImage interface

Additionally, this CL:

adds crop handling to the CPU path (the GPU path handled it but with a bug)

adds a cropRect (to better justify the applyCropRect)

adds a GM to exercise the cropRect

Note: I believe the handling of cropRects and clipping is deeply flawed but, at least, the two paths are consistently flawed now.

TBR=reed@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1882943002

Review URL: https://codereview.chromium.org/1882943002
3 files changed