Fix ambiguity in negate()s.

src/core/Sk4x.h

Index: src/core/Sk4x.h
diff --git a/src/core/Sk4x.h b/src/core/Sk4x.h
index 99ed67d50a35d819c59a5bbb8a7000fe8a689548..ebbc20bdcb82236269b1b08e86e5722ce1a01cfa 100644
--- a/src/core/Sk4x.h
+++ b/src/core/Sk4x.h
@@ -69,7 +69,7 @@ public:
     Sk4x& operator *=(const Sk4x& o) { return (*this = *this * o); }
     Sk4x& operator /=(const Sk4x& o) { return (*this = *this / o); }
 
-    Sk4x negate() const { return Sk4x(0) - *this; }
+    Sk4x negate() const { return Sk4x((T)0) - *this; }
     Sk4x operator -() const { return this->negate(); }
 
     Sk4x rsqrt() const;   // Approximate reciprocal sqrt().