fix SkJumper radial gradient precision

rcp(rsqrt(x)) doesn't have enough precision when x is a coordinate.
(It's fine when x is a color, like in the softlight blend mode.)

Adds a GM to test this.  It used to look quite ugly.

Change-Id: Icec295c2e2f50ae7a5e3e33c62270f632a58f65c
Reviewed-on: https://skia-review.googlesource.com/16914
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/gn/gm.gni b/gn/gm.gni
index 4bcd10389b..2e1c8aff93 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -235,6 +235,7 @@ gm_sources = [
   "$_gm/poly2poly.cpp",
   "$_gm/polygons.cpp",
   "$_gm/quadpaths.cpp",
+  "$_gm/radial_gradient_precision.cpp",
   "$_gm/readpixels.cpp",
   "$_gm/recordopts.cpp",
   "$_gm/rectangletexture.cpp",