Prepare to test all image decode modes to F16

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>