fix issue leecade#1171 - slide alignment issue (leecade#1174)

Slider doesn't calculate offsets correctly when slider isn't full screen because when componentDidUpdate is called and updates the state based on an initState call it uses the return from Dimensions.get('window') instead of the value from props if there is one. The value that should be used is calculated just before the incorrect value is used.

src/index.js

@@ -284,7 +284,7 @@ export default class extends Component {
     }
 
     initState.offset[initState.dir] =
-      initState.dir === 'y' ? height * props.index : width * props.index
+      initState.dir === 'y' ? initState.height * props.index : initState.width * props.index
 
     this.internals = {
       ...this.internals,