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

src/core/Sk2x.h

Index: src/core/Sk2x.h
diff --git a/src/core/Sk2x.h b/src/core/Sk2x.h
index 910476efd7ea75dfbdc1d8a136fcc8dc55b75cad..e9576ded997bacf054cac1e7a8122a0e99497073 100644
--- a/src/core/Sk2x.h
+++ b/src/core/Sk2x.h
@@ -58,6 +58,9 @@ public:
     Sk2x& operator -=(const Sk2x& o) { return (*this = *this - o); }
     Sk2x& operator *=(const Sk2x& o) { return (*this = *this * o); }
 
+    Sk2x negate() const { return Sk2x(0) - *this; }
+    Sk2x operator -() const { return this->negate(); }
+
     Sk2x rsqrt() const;   // Approximate 1/this->sqrt().
     Sk2x  sqrt() const;   // this->multiply(this->rsqrt()) may be faster, but less precise.