Do not use window width; get the width of the current viewport

[fatuhoku]

I use react-native-parallax-view inside of a form-sheet presented modal view on iPad. This means that while the viewport area is 540pt wide, the screen actually is much wider.

At present, the image's width is given by width: screen.width, which is far too wide. this makes the image appear too 'zoomed in' and basically it has the wrong aspect ratio.