Add a clamp stage to SkRasterPipelineBlitter.

src/core/SkSRGB.h

Index: src/core/SkSRGB.h
diff --git a/src/core/SkSRGB.h b/src/core/SkSRGB.h
index 08ba860e4a5df9aeb545d95165a62b37dc969728..31fd4aec5e2d31f005121fc03caddb491ce385b0 100644
--- a/src/core/SkSRGB.h
+++ b/src/core/SkSRGB.h
@@ -28,14 +28,15 @@ static inline Sk4f sk_clamp_0_255(const Sk4f& x) {
     return Sk4f::Min(Sk4f::Max(x, 0.0f), 255.0f);
 }
 
-static inline Sk4i sk_linear_to_srgb(const Sk4f& x) {
+// This should probably only be called from sk_linear_to_srgb() or sk_linear_to_srgb_noclamp().
+// It generally doesn't make sense to work with sRGB floats.
+static inline Sk4f sk_linear_to_srgb_needs_trunc(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.
-
+    // Constants tuned by brute force to minimize (in order of importance) after truncation:
+    //    1) the number of bytes that fail to round trip (0 of 256);
+    //    2) the number of points in [FLT_MIN, 1.0f] that are non-monotonic (0 of ~1 billion);
+    //    3) the number of points halfway between bytes that hit the wrong byte (131 of 255).
     auto rsqrt = x.rsqrt(),
          sqrt  = rsqrt.invert(),
          ftrt  = rsqrt.rsqrt();
@@ -45,8 +46,20 @@ static inline Sk4i sk_linear_to_srgb(const Sk4f& x) {
     auto hi = (-0.0974983f * 255.0f)
             + (+0.687999f  * 255.0f) * sqrt
             + (+0.412999f  * 255.0f) * ftrt;
+    return (x < 0.0048f).thenElse(lo, hi);
+}
+
+static inline Sk4i sk_linear_to_srgb(const Sk4f& x) {
+    Sk4f f = sk_linear_to_srgb_needs_trunc(x);
+    return SkNx_cast<int>(sk_clamp_0_255(f));
+}
 
-    return SkNx_cast<int>(sk_clamp_0_255((x < 0.0048f).thenElse(lo, hi)));
+static inline Sk4i sk_linear_to_srgb_noclamp(const Sk4f& x) {
+    Sk4f f = sk_linear_to_srgb_needs_trunc(x);
+    for (int i = 0; i < 4; i++) {
+        SkASSERTF(0.0f <= f[i] && f[i] < 256.0f, "f[%d] was %g, outside [0,256)\n", i, f[i]);
+    }
+    return SkNx_cast<int>(f);
 }
 
 #endif//SkSRGB_DEFINED