made point lights linear attenuation; also fixed point light depth bug

src/core/SkRadialShadowMapShader.cpp

Index: src/core/SkRadialShadowMapShader.cpp
diff --git a/src/core/SkRadialShadowMapShader.cpp b/src/core/SkRadialShadowMapShader.cpp
index 24b54a5e2be3a1f2271c194b7e2dddbf076e1ff7..29773a63b513f3aaa6eb8fc6be87f92b708f6cb2 100644
--- a/src/core/SkRadialShadowMapShader.cpp
+++ b/src/core/SkRadialShadowMapShader.cpp
@@ -142,7 +142,9 @@ public:
 
             // Modify the input texture coordinates to index into our 1D output
             fragBuilder->codeAppend("float distHere;");
-            fragBuilder->codeAppend("float closestDistHere = 0;");
+
+            // we use a max shadow distance of 2 times the max of width/height
+            fragBuilder->codeAppend("float closestDistHere = 2;");
             fragBuilder->codeAppend("vec2 coords = vMatrixCoord_0_0_Stage0;");
             fragBuilder->codeAppend("coords.y = 0;");
             fragBuilder->codeAppend("vec2 destCoords = vec2(0,0);");