Preventing division by 0 in non-separable blend mode shaders.

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 0cc120dfa7db2104edd36c0240ba55b74c9b99c4..6d893a426d13e9b5d7edddd1b9c707ee7f02a071 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -1100,10 +1100,10 @@ public:
             setLumBody.appendf("\tfloat outLum = %s(outColor);\n", getFunction.c_str());
             setLumBody.append("\tfloat minComp = min(min(outColor.r, outColor.g), outColor.b);\n"
                               "\tfloat maxComp = max(max(outColor.r, outColor.g), outColor.b);\n"
-                              "\tif (minComp < 0.0) {\n"
+                              "\tif (minComp < 0.0 && outLum != minComp) {\n"
                               "\t\toutColor = outLum + ((outColor - vec3(outLum, outLum, outLum)) * outLum) / (outLum - minComp);\n"
                               "\t}\n"
-                              "\tif (maxComp > alpha) {\n"
+                              "\tif (maxComp > alpha && maxComp != outLum) {\n"
                               "\t\toutColor = outLum + ((outColor - vec3(outLum, outLum, outLum)) * (alpha - outLum)) / (maxComp - outLum);\n"
                               "\t}\n"
                               "\treturn outColor;\n");