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

src/core/SkRadialShadowMapShader.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkLights.h"
 #include "SkPoint3.h"
 #include "SkRadialShadowMapShader.h"
@@ skipping 124 matching lines @@
                                                     kInt_GrSLType,
                                                     kDefault_GrSLPrecision,
                                                     "height", &heightUniName);
 
 
             SkString occluder("occluder");
             this->emitChild(0, nullptr, &occluder, args);
 
             // 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);");
             fragBuilder->codeAppendf("float step = 1.0 / %s;", heightUniName);
 
             // assume that we are at 0, 0 light pos
             // TODO use correct light positions
 
             // this goes through each depth value in the final output buffer,
             // basically raycasting outwards, and finding the first collision.
@@ skipping 264 matching lines @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkRadialShadowMapShader)
 SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkRadialShadowMapShaderImpl)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
 
 ///////////////////////////////////////////////////////////////////////////////
 
 #endif