SkLS accepts nullptr for pointer args, handles alpha accurately, has new GM

src/gpu/GrFragmentProcessor.cpp

Index: src/gpu/GrFragmentProcessor.cpp
diff --git a/src/gpu/GrFragmentProcessor.cpp b/src/gpu/GrFragmentProcessor.cpp
index bad7ebe0360cd046262d39b4fa639aeecb874445..fdba610356c2dd74a0c786027809a29b0255f7a0 100644
--- a/src/gpu/GrFragmentProcessor.cpp
+++ b/src/gpu/GrFragmentProcessor.cpp
@@ -143,6 +143,46 @@ sk_sp<GrFragmentProcessor> GrFragmentProcessor::MulOutputByInputAlpha(
                                                              SkXfermode::kDstIn_Mode);
 }
 
+sk_sp<GrFragmentProcessor> GrFragmentProcessor::PremulInput(sk_sp<GrFragmentProcessor> fp) {
+
+    class PremulInputFragmentProcessor : public GrFragmentProcessor {
+    public:
+        PremulInputFragmentProcessor() {
+            this->initClassID<PremulInputFragmentProcessor>();
+        }
+
+        const char* name() const override { return "PremultiplyInput"; }
+
+    private:
+        GrGLSLFragmentProcessor* onCreateGLSLInstance() const override {
+            class GLFP : public GrGLSLFragmentProcessor {
+            public:
+                void emitCode(EmitArgs& args) override {
+                    GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
+
+                    fragBuilder->codeAppendf("%s = %s;", args.fOutputColor, args.fInputColor);
+                    fragBuilder->codeAppendf("%s.rgb *= %s.a;",
+                                             args.fOutputColor, args.fInputColor);
+                }
+            };
+            return new GLFP;
+        }
+
+        void onGetGLSLProcessorKey(const GrGLSLCaps&, GrProcessorKeyBuilder*) const override {}
+
+        bool onIsEqual(const GrFragmentProcessor&) const override { return true; }
+
+        void onComputeInvariantOutput(GrInvariantOutput* inout) const override {
+            inout->premulFourChannelColor();
+        }
+    };
+    if (!fp) {
+        return nullptr;
+    }
+    sk_sp<GrFragmentProcessor> fpPipeline[] = { sk_make_sp<PremulInputFragmentProcessor>(), fp};
+    return GrFragmentProcessor::RunInSeries(fpPipeline, 2);
+}
+
 sk_sp<GrFragmentProcessor> GrFragmentProcessor::MulOutputByInputUnpremulColor(
     sk_sp<GrFragmentProcessor> fp) {