Implement an SkPaint-based image filter

This implements SkPaintImageFilter, and is intended to replace
SkRectShaderImageFilter. By allowing a paint and not just a
shader as input, this allows consumers to control dithering.

BUG=skia:4780
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1556553002

Review URL: https://codereview.chromium.org/1556553002
11 files changed