hack on linear gradient

Am I going nuts or can we get this down to just adds and converts in the loop?

#floats #n9
BUG=skia:3592
CQ_INCLUDE_TRYBOTS=client.skia.android:Test-Android-Nexus9-TegraK1-Arm64-Release-Trybot

Review URL: https://codereview.chromium.org/1008973004