Add clamp to sk_linear_to_srgb, reorder instructions

src/core/SkSRGB.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSRGB_DEFINED
 #define SkSRGB_DEFINED
 
 #include "SkNx.h"
 
 /** Components for building our canonical sRGB -> linear and linear -> sRGB transformations.
  *
  *  Current best practices:
  *      - for sRGB -> linear, lookup R,G,B in sk_linear_from_srgb;
  *      - for linear -> sRGB, call sk_linear_to_srgb() for R,G,B;
  *      - the alpha channel is linear in both formats, needing at most *(1/255.0f) or *255.0f.
  *
  *  sk_linear_to_srgb() will run a little faster than usual when compiled with SSE4.1+.
  */
 
 extern const float sk_linear_from_srgb[256];
 
+static inline Sk4f sk_clamp_0_255(const Sk4f& x) {
+    // The order of the arguments is important here.  We want to make sure that NaN
+    // clamps to zero.  Note that max(NaN, 0) = 0, while max(0, NaN) = NaN.
+    return Sk4f::Min(Sk4f::Max(x, 0.0f), 255.0f);
+}
+
 static inline Sk4i sk_linear_to_srgb(const Sk4f& x) {
     // Approximation of the sRGB gamma curve (within 1 when scaled to 8-bit pixels).
     //
     // Tuned by brute force to minimize the number of bytes that fail to round trip,
     // here 0 (of 256), and then to minimize the number of points halfway between bytes
     // (in linear space) that fail to hit the right byte, here 131 (of 255), and to
     // minimize the number of monotonicity regressions over the range [0,1], here 0.
 
     auto rsqrt = x.rsqrt(),
          sqrt  = rsqrt.invert(),
          ftrt  = rsqrt.rsqrt();
 
     auto lo = (13.0471f * 255.0f) * x;
 
     auto hi = (-0.0974983f * 255.0f)
             + (+0.687999f  * 255.0f) * sqrt
             + (+0.412999f  * 255.0f) * ftrt;
 
-    return SkNx_cast<int>( (x < 0.0048f).thenElse(lo, hi) );
+    return SkNx_cast<int>(sk_clamp_0_255((x < 0.0048f).thenElse(lo, hi)));
 }
 
 #endif//SkSRGB_DEFINED