Support WebXR in a self-contained way

[zachernuk]

Because of WebXR's history as targeting a separate display device (in the case of a desktop computer connected to an HMD), it requires a separate requestAnimationFrame handler to manage the frame rate - even in modern, standalone webXR devices. This update localizes the changes required to webgl.js by dynamically patching the containing Benchmark instance, and adding only the additional parameters required to do the current webGL tests within webXR.