Fix handling of offsets and transparent colors in gradients

- Colors in gradient can have their own alpha value, so do not use the
one from the main paint color when doing the compositing.
- Handle offsets as the platform.

Bug: 112759140
Test: RenderTests.tesGradientColors added
Change-Id: Id864fe5d811b796849e5eb85d5abc7c5a32848d3
(cherry picked from commit c5b57482f99acb9c20fd559fc51803d97d8043b5)