support image-subsets in serialization

A follow-on API change *could* be to extend the SerialProcs to pass subset information
up to the client, in case they want to handle the subsetting step themselves.

Bug: skia:7983
Change-Id: I36d3f1ce439886384495485c3be3c591d611a135
Reviewed-on: https://skia-review.googlesource.com/130543
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
9 files changed