[VR] Use WebVR if available and headset attached

[satoshinm]

If the browser supports WebVR, and if it detects a valid head-mounted display, then use WebVR to render, without applying the barrel distortion shader effects (WebVR does this itself).

Continuation of #63 VR

Major impediment of doing this myself is lack of WebVR support (only have an Oculus Rift DK, which doesn't support WebVR, not even on Windows. WebVR API emulation? https://chrome.google.com/webstore/detail/webvr-api-emulation/gbdnpaebafagioggnhkacnaaahpiefil)