add SkNx::abs(), for now only implemented for Sk4f

tests/SkNxTest.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 "Sk4px.h"
 #include "SkNx.h"
 #include "SkRandom.h"
@@ skipping 198 matching lines @@
 
 DEF_TEST(SkNx_cast, r) {
     Sk4f fs(-1.7f, -1.4f, 0.5f, 1.9f);
     Sk4i is = SkNx_cast<int>(fs);
 
     REPORTER_ASSERT(r, is.kth<0>() == -1);
     REPORTER_ASSERT(r, is.kth<1>() == -1);
     REPORTER_ASSERT(r, is.kth<2>() ==  0);
     REPORTER_ASSERT(r, is.kth<3>() ==  1);
 }
+
+DEF_TEST(SkNx_abs, r) {
+    auto fs = Sk4f(0.0f, -0.0f, 2.0f, -4.0f).abs();
+    REPORTER_ASSERT(r, fs.kth<0>() == 0.0f);
+    REPORTER_ASSERT(r, fs.kth<1>() == 0.0f);
+    REPORTER_ASSERT(r, fs.kth<2>() == 2.0f);
+    REPORTER_ASSERT(r, fs.kth<3>() == 4.0f);
+}