Move GPU specific ImageFilter context factories to src/gpu/

This removes ganesh- and graphite- specific code out of
src/core/SkImageFilterTypes

Bug: skia:14317
Change-Id: I6c4a62ede656c97e83663dc8c7cffa1803005eea
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/717976
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
12 files changed