Lazily evaluate disjoint timer extension

[mvaligursky]

This extension is not typically needed in builds without GPU profiling, so evaluate the extension lazily to avoid the cost at start up.

[willeastcott]

Shouldn't we have a generic approach to lazily wrangling all WebGL extensions? I had considered this in the past but never got around to it. Obviously, some extensions will be required in all circumstances....but it still might be worth doing this across the board for consistency.

[mvaligursky]

Ideally yes, but that's a larger job. I addressed a specific bottleneck in the app I profiled. We only have 2 extensions on WebGl 2, and the other one is often required, so this is the only saving on WebGL2 here. WebGl 1 is a separate story, with lots of extensions, but I didn't go there.