linear -> sRGB: use fast approximate sqrt()

src/core/SkPM4fPriv.h

Index: src/core/SkPM4fPriv.h
diff --git a/src/core/SkPM4fPriv.h b/src/core/SkPM4fPriv.h
index 9650646d95d3bcfa012fede18fd38bb7fd477e65..bf5ff5a1ad44b4914caf36d005653dad4e20c5da 100644
--- a/src/core/SkPM4fPriv.h
+++ b/src/core/SkPM4fPriv.h
@@ -41,7 +41,7 @@ static inline Sk4f srgb_to_linear(const Sk4f& 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) {