Add F16Norm mode to viewer

Change-Id: If7b84731bce9ba8f1034c974fc7852ee304b499d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/242478
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed