Prepare to test all image decode modes to F16

I've only been running F16 tests in "colorImage" mode.
We really should be running our "image" tests to F16 as well - so
they can be tested with scaling, subsets, etc.

This CL fixes bugs so that we can enable those tests.

BUG=skia:

Change-Id: I8137eb4fce7ea12f2c9d233a029d946d4a63e6b0
Reviewed-on: https://skia-review.googlesource.com/7801
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
5 files changed