linear -> sRGB: use fast approximate sqrt()

src/core/SkPM4fPriv.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 SkPM4fPriv_DEFINED
 #define SkPM4fPriv_DEFINED
 
@@ skipping 23 matching lines @@
 
 static inline Sk4f to_4f_rgba(uint32_t b4) {
     return swizzle_rb_if_bgra(to_4f(b4));
 }
 
 static inline Sk4f srgb_to_linear(const Sk4f& s4) {
     return set_alpha(s4 * s4, get_alpha(s4));
 }
 
 static inline Sk4f linear_to_srgb(const Sk4f& l4) {
-    return set_alpha(l4.sqrt(), get_alpha(l4));
+    return set_alpha(l4.rsqrt().invert(), get_alpha(l4));
 }
 
 static inline float srgb_to_linear(float x) {
     return x * x;
 }
 
 static inline float linear_to_srgb(float x) {
     return sqrtf(x);
 }
 
@@ skipping 16 matching lines @@
 
 static inline uint32_t Sk4f_toL32(const Sk4f& x4) {
     return to_4b(x4 * Sk4f(255) + Sk4f(0.5f));
 }
 
 static inline uint32_t Sk4f_toS32(const Sk4f& x4) {
     return to_4b(linear_to_srgb(x4) * Sk4f(255) + Sk4f(0.5f));
 }
 
 #endif