Improve naive SkColorXform to half floats

src/effects/gradients/Sk4fGradientPriv.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef Sk4fGradientPriv_DEFINED
 #define Sk4fGradientPriv_DEFINED
 
@@ skipping 125 matching lines @@
 template <ApplyPremul premul>
 struct DstTraits<DstType::F16, premul> {
     using PM   = PremulTraits<premul>;
     using Type = uint64_t;
 
     static Sk4f load(const SkPM4f& c) {
         return c.to4f();
     }
 
     static void store(const Sk4f& c, Type* dst) {
-        *dst = SkFloatToHalf_finite(PM::apply(c));
+        SkFloatToHalf_finite(PM::apply(c)).store(dst);
     }
 
     static void store(const Sk4f& c, Type* dst, int n) {
-        sk_memset64(dst, SkFloatToHalf_finite(PM::apply(c)), n);
+        uint64_t color;
+        SkFloatToHalf_finite(PM::apply(c)).store(&color);
+        sk_memset64(dst, color, n);
     }
 
     static void store4x(const Sk4f& c0, const Sk4f& c1,
                         const Sk4f& c2, const Sk4f& c3,
                         Type* dst) {
         store(c0, dst + 0);
         store(c1, dst + 1);
         store(c2, dst + 2);
         store(c3, dst + 3);
     }
@@ skipping 25 matching lines @@
         store(c0, dst + 0);
         store(c1, dst + 1);
         store(c2, dst + 2);
         store(c3, dst + 3);
     }
 };
 
 } // anonymous namespace
 
 #endif // Sk4fGradientPriv_DEFINED