Sk4px: Difference and Exclusion

src/opts/Sk4px_SSE2.h

Index: src/opts/Sk4px_SSE2.h
diff --git a/src/opts/Sk4px_SSE2.h b/src/opts/Sk4px_SSE2.h
index 55e1a69135d561a42d51f5dd2e456d7c1d05564c..9ba510347ec89808f972cd3a97e78406bea478ff 100644
--- a/src/opts/Sk4px_SSE2.h
+++ b/src/opts/Sk4px_SSE2.h
@@ -76,3 +76,12 @@ inline Sk4px Sk4px::Load2Alphas(const SkAlpha a[2]) {
     uint32_t as = *(const uint16_t*)a;   // Aa -> Aa00
     return Load4Alphas((const SkAlpha*)&as);
 }
+
+inline Sk4px Sk4px::zeroColors() const {
+    return Sk16b(_mm_and_si128(_mm_set1_epi32(0xFF << SK_A32_SHIFT), this->fVec));
+}
+
+inline Sk4px Sk4px::zeroAlphas() const {
+    // andnot(a,b) == ~a & b
+    return Sk16b(_mm_andnot_si128(_mm_set1_epi32(0xFF << SK_A32_SHIFT), this->fVec));
+}