Make Sk4f(float) constructor explicit.

src/core/Sk4x.h

 #ifndef Sk4x_DEFINED
 #define Sk4x_DEFINED
 
 #include "SkTypes.h"
 
 #define SK4X_PREAMBLE 1
     #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
         #include "Sk4x_sse.h"
     #elif defined(SK_ARM_HAS_NEON)
         #include "Sk4x_neon.h"
     #else
         #include "Sk4x_portable.h"
     #endif
 #undef SK4X_PREAMBLE
 
 template <typename T> class Sk4x;
 typedef Sk4x<float>   Sk4f;
 typedef Sk4x<int32_t> Sk4i;
 
 // Some Sk4x methods are implemented only for Sk4f or Sk4i.
 // They might be unavailable, really slow, or just a bad idea.
 // Talk to mtklein if you find yourself unable to link and
 // really need one of those methods.
 
 template <typename T> class Sk4x {
 public:
     Sk4x();  // Uninitialized; use Sk4x(0) for zero.
-    /*implicit*/ Sk4x(T);  // Same as Sk4x(T,T,T,T);
+    explicit Sk4x(T);  // Same as Sk4x(T,T,T,T);
     Sk4x(T, T, T, T);
 
     Sk4x(const Sk4x&);
     Sk4x& operator=(const Sk4x&);
 
     static Sk4x Load       (const T[4]);
     static Sk4x LoadAligned(const T[4]);
 
     void store       (T[4]) const;
     void storeAligned(T[4]) const;
@@ skipping 54 matching lines @@
 
 #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
     #include "Sk4x_sse.h"
 #elif defined(SK_ARM_HAS_NEON)
     #include "Sk4x_neon.h"
 #else
     #include "Sk4x_portable.h"
 #endif
 
 #endif//Sk4x_DEFINED