Image filters: fix srcOffset handling in asFragmentProcessor() path.

Image filters: fix srcOffset handling in asFragmentProcessor() path.

Filters such as SkMatrixConvolutionImageFilter which use the
asFragmentProcessor() path were not correctly handling srcOffset.
It is correctly applied to the bounds, but the srcRect and dstRect
were computed from the pre-offset bounds.

The fix is to move them to just above where they're used in drawing.

Note: this change adds a new test case to the imagefiltersgraph GM,
so it will have to be rebaselined post-landing.

BUG=skia:4855
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1637443003

Review URL: https://codereview.chromium.org/1637443003