linear -> sRGB: use fast approximate sqrt()

src/effects/gradients/Sk4fLinearGradient.cpp

Index: src/effects/gradients/Sk4fLinearGradient.cpp
diff --git a/src/effects/gradients/Sk4fLinearGradient.cpp b/src/effects/gradients/Sk4fLinearGradient.cpp
index 090bdaa83555ab5dd6a4cf3b279416c075393da1..ca0ca6da60b7b1dcc71672d67ee7bd1b91248e5a 100644
--- a/src/effects/gradients/Sk4fLinearGradient.cpp
+++ b/src/effects/gradients/Sk4fLinearGradient.cpp
@@ -53,7 +53,12 @@ void ramp<DstType::S32, ApplyPremul::False>(const Sk4f& c, const Sk4f& dc, SkPMC
     Sk4x4f        c4x = Sk4x4f::Transpose(c, c + dc, c + dc * 2, c + dc * 3);
 
     while (n >= 4) {
-        const Sk4x4f cx4s32 = { c4x.r.sqrt(), c4x.g.sqrt(), c4x.b.sqrt(), c4x.a };
+        const Sk4x4f cx4s32 = {
+            c4x.r.rsqrt().invert(),
+            c4x.g.rsqrt().invert(),
+            c4x.b.rsqrt().invert(),
+            c4x.a
+        };
         cx4s32.transpose((uint8_t*)dst);
 
         c4x.r += dc4x.r;