Improvements to readpixels GM

We don't support unpremultiplied images. Therefore:
- Don't test unpremul source images.
- After doing an unpremultiplying read, make sure to premul before drawing

For this to work with F16, add support for F16 sources to
SkColorSpaceXform.

Public API change is comments-only.
TBR=reed@google.com

BUG=skia:

Change-Id: Ie05b58231e99ca88cd7792b65ffbb4f390b01726
Reviewed-on: https://skia-review.googlesource.com/9900
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
3 files changed