shadowOpacity has no effect
I am trying to use the shadowOpacity style property, but it has no effect. When rendered to Sketch, the shadow always shows with Alpha 100.

**Expected behavior:**
Setting:
```
style={{
  shadowOpacity: 0.5,
  shadowOffset: {
    width: 10,
    height: 14
  }
}}
```
**Observed behavior:**
...should show as a shadow with 50% opacity in Sketch.

**How to reproduce:**
From the example project linked below, render `examples/color` to Sketch. I've added a shadow to reproduce the issue.
https://github.com/CLL80/react-sketchapp/tree/issue/shadowOpacity
https://github.com/CLL80/react-sketchapp/blob/issue/shadowOpacity/examples/colors/src/main.js

**Sketch version:**
43.2
