Revert "Remove ambient clamp hack for analytic shadows."

This reverts commit d75b34394b5c53b0a5e8144dd28f956fbc8b90b2.

Reason for revert: Breaking GMs

Original change's description:
> Remove ambient clamp hack for analytic shadows.
> 
> The hack was added to make analytic ambient shadows match the geometric
> ones exactly. Removing it does produce a slight difference, but it's
> only visible at full black values (and then, only slightly) and this
> makes room to pass a blend parameter for falloff.
> 
> Also fixes an issue with filling out arcs in the shadow tesselator.
> 
> Bug: skia:7486
> Change-Id: If78d0104df7e18f4320c1658af75576f6a4a8c73
> Reviewed-on: https://skia-review.googlesource.com/122580
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Jim Van Verth <jvanverth@google.com>

TBR=jvanverth@google.com,bsalomon@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: skia:7486
Change-Id: If0a20a358759c08a3a475d60be54a3d62ff7b6ad
Reviewed-on: https://skia-review.googlesource.com/123120
Reviewed-by: Jim Van Verth <jvanverth@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/gpu/effects/GrShadowGeoProc.cpp b/src/gpu/effects/GrShadowGeoProc.cpp
index 9f84aa2ae7..7eddaa41d8 100644
--- a/src/gpu/effects/GrShadowGeoProc.cpp
+++ b/src/gpu/effects/GrShadowGeoProc.cpp
@@ -45,7 +45,7 @@ public:
         fragBuilder->codeAppend("half d = length(shadowParams.xy);");
         fragBuilder->codeAppend("half distance = shadowParams.z * (1.0 - d);");
 
-        fragBuilder->codeAppend("half factor = 1.0 - clamp(distance, 0.0, 1.0);");
+        fragBuilder->codeAppend("half factor = 1.0 - clamp(distance, 0.0, shadowParams.w);");
         fragBuilder->codeAppend("factor = exp(-factor * factor * 4.0) - 0.018;");
         fragBuilder->codeAppendf("%s = half4(factor);",
                                  args.fOutputCoverage);