Refactor Sk2x<T> + Sk4x<T> into SkNf<N,T> and SkNi<N,T>

src/opts/SkPMFloat_none.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
-inline SkPMFloat& SkPMFloat::operator=(const SkPMFloat& that) {
-    for (int i = 0; i < 4; i++) { fColor[i] = that.fColor[i]; }
-    return *this;
-}
-
 inline SkPMFloat::SkPMFloat(SkPMColor c) {
     *this = SkPMFloat::FromARGB(SkGetPackedA32(c),
                                 SkGetPackedR32(c),
                                 SkGetPackedG32(c),
                                 SkGetPackedB32(c));
     SkASSERT(this->isValid());
 }
 
 inline SkPMColor SkPMFloat::trunc() const {
     return SkPackARGB32(this->a(), this->r(), this->g(), this->b());
@@ skipping 34 matching lines @@
 }
 
 inline void SkPMFloat::ClampTo4PMColors(
         const SkPMFloat& a, const SkPMFloat& b, const SkPMFloat&c, const SkPMFloat& d,
         SkPMColor colors[4]) {
     colors[0] = a.clamped();
     colors[1] = b.clamped();
     colors[2] = c.clamped();
     colors[3] = d.clamped();
 }