Fixed resolution on mobile phones

On mobile, resolution was incorrect due to devicePixelRatio != 1
I would simplify this whole PR to just this:

```javascript
renderTarget = new THREE.WebGLRenderTarget( ( HMD.hResolution * distScale / 2 ) * renderer.devicePixelRatio, ( HMD.vResolution * distScale ) * renderer.devicePixelRatio, RTParams );
```