add auto SkPMFloat <-> Sk4f conversion

src/core/SkPMFloat.h

 #ifndef SkPM_DEFINED
 #define SkPM_DEFINED
 
 #include "SkTypes.h"
 #include "SkColor.h"
+#include "Sk4x.h"
 
 // A pre-multiplied color in the same order as SkPMColor storing each component as a float.
 struct SK_STRUCT_ALIGN(16) SkPMFloat {
     float fColor[4];
 
+    // Normal POD copies and do-nothing initialization.
+    SkPMFloat()                            = default;
+    SkPMFloat(const SkPMFloat&)            = default;
+    SkPMFloat& operator=(const SkPMFloat&) = default;
+
+    // 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; }
 
@@ skipping 12 matching lines @@
 
 #if 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