Add Load2/store2 to Sk4x (dumb impl for now)

src/core/Sk4x.h

Index: src/core/Sk4x.h
diff --git a/src/core/Sk4x.h b/src/core/Sk4x.h
index 0b9796db9b9bb1e42ee23606e27f3f9deeb4952a..401bbd857b1ce176c3820ecb8d87f0a15180019c 100644
--- a/src/core/Sk4x.h
+++ b/src/core/Sk4x.h
@@ -37,6 +37,18 @@ public:
     void store       (T[4]) const;
     void storeAligned(T[4]) const;
 
+    // Experimental!
+    static Sk4x Load2(const T src[2]) {
+        const T padded[4] = { src[0], src[1], 0, 0 };
+        return Load(padded);
+    }
+    void store2(T dst[2]) const {
+        T padded[4];
+        this->store(padded);
+        dst[0] = padded[0];
+        dst[1] = padded[1];
+    }
+
     template <typename Dst> Dst reinterpret() const;
     template <typename Dst> Dst        cast() const;