Fix color xforms for Index8 bmps

Thanks to Gold for catching this.

BUG=skia:4895

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4426

Change-Id: Icc816d933e9e2fd312858a5430fa21a47722563e
Reviewed-on: https://skia-review.googlesource.com/4426
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
5 files changed