Simplify ColorFilter in the add-free case.

src/effects/SkColorMatrixFilter.cpp

Index: src/effects/SkColorMatrixFilter.cpp
diff --git a/src/effects/SkColorMatrixFilter.cpp b/src/effects/SkColorMatrixFilter.cpp
index 82bd45b73ddd759bb0d19ec1bc56f9f4518a6bfa..32cb3d91616be67151779375836790b75899ad68 100644
--- a/src/effects/SkColorMatrixFilter.cpp
+++ b/src/effects/SkColorMatrixFilter.cpp
@@ -17,6 +17,13 @@ static SkScalar byte_to_scale(U8CPU byte) {
 }
 
 sk_sp<SkColorFilter> SkColorMatrixFilter::MakeLightingFilter(SkColor mul, SkColor add) {
+    const SkColor opaqueAlphaMask = SK_ColorBLACK;
+    // omit the alpha and compare only the RGB values
+    if (0 == (add & ~opaqueAlphaMask)) {
+        return SkColorFilter::MakeModeFilter(mul | opaqueAlphaMask,
+                                             SkXfermode::Mode::kModulate_Mode);
+    }
+
     SkColorMatrix matrix;
     matrix.setScale(byte_to_scale(SkColorGetR(mul)),
                     byte_to_scale(SkColorGetG(mul)),