Async resource loading

[randomPoison]

Support loading a processing the following resource types asynchronously:

• [x] Collada meshes (.dae).
• [x] Polygon materials (.material).

The goal here is to recreate as much of the existing functionality as is reasonable, but not to make any major additions if we can avoid it.

[randomPoison]

Now that the async system has been fleshed out more, the work here is fairly straightforward. We need to copy the old, synchronous code for loading .wav files and add a function to async::resource to run the sync code within scheduler::start().

[randomPoison]

I'm removing the requirement for loading .wav files since it seems unlikely that I'll redo the audio functionality any time soon.