Default backgroundColor color cant be set without a setState()
I'm unable to set a backgroundColor to something other then 'transparent' when loading a SketchField

Even when I change your example main. (Line:354)
```             
backgroundColor={
  this.state.fillWithBackgroundColor
    ? this.state.backgroundColor
    : 'transparent'
}
```

Too
```
backgroundColor={
  this.state.fillWithBackgroundColor
    ? this.state.backgroundColor
    : '#F44E3B'
}
```

And set the default state of backgroundColor to '#F44E3B' it's still transparent it only changes when I press the switch and call a `this.setState()`