Add sqrt() and rsqrt() to Sk4f.

src/core/Sk4x.h

Index: src/core/Sk4x.h
diff --git a/src/core/Sk4x.h b/src/core/Sk4x.h
index 058c40038037f0516efae2ccc8dfead9bb749ef4..b01b6f16484d519d1ca58a15a1fe8c5e83f5c992 100644
--- a/src/core/Sk4x.h
+++ b/src/core/Sk4x.h
@@ -50,6 +50,9 @@ public:
     Sk4x multiply(const Sk4x&) const;
     Sk4x   divide(const Sk4x&) const;
 
+    Sk4x rsqrt() const;   // Approximate reciprocal sqrt().
+    Sk4x  sqrt() const;   // this->multiply(this->rsqrt()) may be faster, but less precise.
+
     Sk4i            equal(const Sk4x&) const;
     Sk4i         notEqual(const Sk4x&) const;
     Sk4i         lessThan(const Sk4x&) const;