Partially defer SkImage_Gpu

One of SkImageCacherator, GrBitmapTextureMaker, GrImageTextureMaker, GrTextureAdjuster, GrTextureProducer or SkImage has to take the first step. This is probably the least odd of the options.

Change-Id: Ie167034553451f4b3633a5a1548dbd4d75839b3d
Reviewed-on: https://skia-review.googlesource.com/9488
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
18 files changed