Change behavior of custom image serial/deserial

New behavior is to *always* call the client's deserial image proc for all data.
This allows the client to make decisions even on "std" image data like PNG.

The change also means that if there is no client deserial image proc, Skia will
still attempt to create an image from the data, even if it was written by a
custom serial proc.

Bug: skia:7706
Change-Id: Ia58bdd10b86d497f02187082c6373c029e9c8293
Reviewed-on: https://skia-review.googlesource.com/113302
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Reed <reed@google.com>
6 files changed