Update SkPMFloat API a bit.

src/core/SkPMFloat.h

 #ifndef SkPM_DEFINED
 #define SkPM_DEFINED
 
 #include "SkTypes.h"
 #include "SkColor.h"
 #include "Sk4x.h"
 
-// A pre-multiplied color storing each component as a float in the range [0, 255].
+// A pre-multiplied color storing each component in the same order as SkPMColor,
+// but as a float in the range [0, 255].
 class SK_STRUCT_ALIGN(16) SkPMFloat {
 public:
-    // Normal POD copies and do-nothing initialization.
-    SkPMFloat()                            = default;
-    SkPMFloat(const SkPMFloat&)            = default;
-    SkPMFloat& operator=(const SkPMFloat&) = default;
+    static SkPMFloat FromPMColor(SkPMColor c) { return SkPMFloat(c); }
+    static SkPMFloat FromARGB(float a, float r, float g, float b) { return SkPMFloat(a,r,g,b); }
+
+    explicit SkPMFloat(SkPMColor);
+    SkPMFloat(float a, float r, float g, float b) {
+        // TODO: faster when specialized?
+        fColor[SK_A32_SHIFT / 8] = a;
+        fColor[SK_R32_SHIFT / 8] = r;
+        fColor[SK_G32_SHIFT / 8] = g;
+        fColor[SK_B32_SHIFT / 8] = b;
+    }
+
+    // Uninitialized.
+    SkPMFloat() {}
+
+    // Copy and assign are fastest if we remind the compiler we work best as Sk4f.
+    SkPMFloat(const SkPMFloat& that) { Sk4f(that).storeAligned(fColor); }
+    SkPMFloat& operator=(const SkPMFloat& that) {
+        Sk4f(that).storeAligned(fColor);
+        return *this;
+    }
 
     // Freely autoconvert between SkPMFloat and Sk4f.
     /*implicit*/ SkPMFloat(const Sk4f& fs) { fs.storeAligned(fColor); }
     /*implicit*/ operator Sk4f() const { return Sk4f::LoadAligned(fColor); }
 
     float a() const { return fColor[SK_A32_SHIFT / 8]; }
     float r() const { return fColor[SK_R32_SHIFT / 8]; }
     float g() const { return fColor[SK_G32_SHIFT / 8]; }
     float b() const { return fColor[SK_B32_SHIFT / 8]; }
 
-    void setA(float val) { fColor[SK_A32_SHIFT / 8] = val; }
-    void setR(float val) { fColor[SK_R32_SHIFT / 8] = val; }
-    void setG(float val) { fColor[SK_G32_SHIFT / 8] = val; }
-    void setB(float val) { fColor[SK_B32_SHIFT / 8] = val; }
-
-    void set(SkPMColor);
-
     // get() and clamped() round component values to the nearest integer.
     SkPMColor     get() const;  // May SkASSERT(this->isValid()).  Some implementations may clamp.
     SkPMColor clamped() const;  // Will clamp all values to [0, 255].  Then may assert isValid().
 
     bool isValid() const {
         return this->a() >= 0 && this->a() <= 255
             && this->r() >= 0 && this->r() <= this->a()
             && this->g() >= 0 && this->g() <= this->a()
             && this->b() >= 0 && this->b() <= this->a();
     }
 
 private:
-    // We mirror SkPMColor order only to make set()/get()/clamped() as fast as possible.
     float fColor[4];
 };
 
 #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSSE3
     #include "../opts/SkPMFloat_SSSE3.h"
 #elif SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
     #include "../opts/SkPMFloat_SSE2.h"
 #elif defined(__ARM_NEON__)
     #include "../opts/SkPMFloat_neon.h"
 #else
     #include "../opts/SkPMFloat_none.h"
 #endif
 
 #endif//SkPM_DEFINED