[includes] Remove link between SkImage_Base.h -> GrBackendSurface.h

This involves enforcing IWYU on SkImage_Base.h and SkGr.*, as well
as moving a function from SkImage_Base.h to SkImage.cpp.

Change-Id: I680d30a47a6e074237dc82a87c3f1d3830a9b485
Bug: skia:13052
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/608016
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
48 files changed