Slightly generalize GPU hard stop gradient implementation.

With this CL we handle single off-center hardstop gradients.

BUG=chromium:543625

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3303

Change-Id: Ic754e87469475ce15865c54055b8ed492e1d826d
Reviewed-on: https://skia-review.googlesource.com/3303
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed