Add sk_float_rsqrt with SSE + NEON fast paths.

src/core/SkPoint.cpp

Index: src/core/SkPoint.cpp
diff --git a/src/core/SkPoint.cpp b/src/core/SkPoint.cpp
index bf3affaaf549d10c155582b65fbfaf71bb874833..d44ec403c79755ac2d0d225a438dcdb39b724589 100644
--- a/src/core/SkPoint.cpp
+++ b/src/core/SkPoint.cpp
@@ -163,7 +163,7 @@ bool SkPoint::setLength(float x, float y, float length) {
 
     float scale;
     if (SkScalarIsFinite(mag2)) {
-        scale = length / sk_float_sqrt(mag2);
+        scale = length * sk_float_rsqrt(mag2);

[reed1, 2013/11/07 22:28:44]

not prepared to land this, until we know more about the assumption of *all* of
our callers.

consider adding setFastLength() for now?

     } else {
         // our mag2 step overflowed to infinity, so use doubles instead.
         // much slower, but needed when x or y are very large, other wise we