Consolidate details of lazy proxy creation for promise image textures.

The SkSpecialImage helper is now function-local and *is* the lazy
instantiation callback rather than coordinating with one in the
promise SkImage factory functions.

Bug: skia:
Change-Id: I4883618ef440d09a9ac7959cef254e6f5a49aa55
Reviewed-on: https://skia-review.googlesource.com/c/175822
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed