Implement scaling in SkCodecImageGenerator

Plumb calls down to SkCodec.

Add a gm

Change-Id: I16da24eb739295ab72f487df02f19968151443f3
Reviewed-on: https://skia-review.googlesource.com/6287
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
4 files changed