Add SkImage_Base API to access planar data

In order to test YUV promise images we need to be able to access the planes.

Split out of: https://skia-review.googlesource.com/c/skia/+/151983 (Test YUV images in DDL)

Bug: skia:7903
Change-Id: If232a5fdeb90833da6db727b1997a3c744bcd9dd
Reviewed-on: https://skia-review.googlesource.com/156140
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed