Avoid unbounded listener growth on SkImage_Lazy when textures are purged

Generalizes the system used on SkPathRef where a GrTexture's key
destructor signals that a listener on the image can be removed via
the unique key custom data.

Removes texturesAreCacheable() from SkImageGenerator. This was used to
prevent unbounded growth in a narrow situation related to
GrBackendTextureImageGenerator.


Change-Id: I3c605da099acfac94751e793331e356a0979d359
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/274038
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
20 files changed