Improve naive SkColorXform to half floats

src/core/SkMipMap.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkMipMap.h"
 #include "SkBitmap.h"
 #include "SkColorPriv.h"
@@ skipping 70 matching lines @@
         return (uint8_t)x;
     }
 };
 
 struct ColorTypeFilter_F16 {
     typedef uint64_t Type; // SkHalf x4
     static Sk4f Expand(uint64_t x) {
         return SkHalfToFloat_finite(x);
     }
     static uint64_t Compact(const Sk4f& x) {
-        return SkFloatToHalf_finite(x);
+        uint64_t r;
+        SkFloatToHalf_finite(x).store(&r);
+        return r;
     }
 };
 
 template <typename T> T add_121(const T& a, const T& b, const T& c) {
     return a + b + b + c;
 }
 
 template <typename T> T shift_right(const T& x, int bits) {
     return x >> bits;
 }
@@ skipping 523 matching lines @@
         return false;
     }
     if (index > fCount - 1) {
         return false;
     }
     if (levelPtr) {
         *levelPtr = fLevels[index];
     }
     return true;
 }