Add async rescale and read APIs to SkImage.

These function the same as the already existing
SkSurface APIs.

Bug: skia:10431

Change-Id: I4f1e842d8d4b72ee27bae5f8a85e499e130d420c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299281
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
22 files changed