Require Sk4f::toBytes() clamps

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 190 matching lines @@
         SkPreMultiplyColor(0x00000000),
     };
     auto packed = Sk4px::Load4(colors);
 
     auto wideLo = packed.widenLo(),
          wideHi = packed.widenHi(),
          wideLoHi    = packed.widenLoHi(),
          wideLoHiAlt = wideLo + wideHi;
     REPORTER_ASSERT(r, 0 == memcmp(&wideLoHi, &wideLoHiAlt, sizeof(wideLoHi)));
 }
+
+DEF_TEST(Sk4f_toBytes, r) {
+    uint8_t bytes[4];
+
+    // toBytes truncates, not rounds.
+    Sk4f(0.7f).toBytes(bytes);
+    REPORTER_ASSERT(r, bytes[0] == 0);
+
+    // Clamping edge cases.
+    Sk4f(-2.0f, -0.7f, 255.9f, 256.0f).toBytes(bytes);
+    REPORTER_ASSERT(r, bytes[0] == 0);
+    REPORTER_ASSERT(r, bytes[1] == 0);
+    REPORTER_ASSERT(r, bytes[2] == 255);
+    REPORTER_ASSERT(r, bytes[3] == 255);
+}