Sk2x/Sk4x: add negate(), operator -()

src/core/Sk4x.h

Index: src/core/Sk4x.h
diff --git a/src/core/Sk4x.h b/src/core/Sk4x.h
index 777927fb773620239415587dcf3e41f43537b397..99ed67d50a35d819c59a5bbb8a7000fe8a689548 100644
--- a/src/core/Sk4x.h
+++ b/src/core/Sk4x.h
@@ -69,6 +69,9 @@ 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 operator -() const { return this->negate(); }
+
     Sk4x rsqrt() const;   // Approximate reciprocal sqrt().
     Sk4x  sqrt() const;   // this->multiply(this->rsqrt()) may be faster, but less precise.