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

tests/Sk2xTest.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Test.h"
 #include "Sk2x.h"
 
@@ skipping 33 matching lines @@
     REPORTER_ASSERT(r, eq(a + d, 6,   9));
     REPORTER_ASSERT(r, eq(a - d, 2,  -1));
     REPORTER_ASSERT(r, eq(a * d, 8,  20));
 
     REPORTER_ASSERT(r, nearly_eq(0.001, a.rsqrt(), 0.5, 0.5));
     REPORTER_ASSERT(r, eq(a.sqrt(), 2, 2));
 
     REPORTER_ASSERT(r, eq(Sk2x<T>::Min(a, d), 2, 4));
     REPORTER_ASSERT(r, eq(Sk2x<T>::Max(a, d), 4, 5));
 
+    REPORTER_ASSERT(r, eq(-d, -2, -5));
+
     // A bit of both.
     a += d;
     a *= d;
     a -= d;
     REPORTER_ASSERT(r, eq(a, 10, 40));
 }
 
 DEF_TEST(Sk2f, r) { test< float>(r); }
 DEF_TEST(Sk2d, r) { test<double>(r); }