Parsing
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Entered LDraw models are parsed line by line, and a three is constructed for the whole model, consisting of BrickSpec and children.
At the end of each part parsing, all Subpart specs are populated, that is, the subparts are loaded.
Loading works like this: a proxy is immediately returned, and an asyncronous request is made. The part 
will support a list of waiters, in the form of a list callbacks, and this callbacks will be called when all subparts are ready and loaded.
The effect of this "lazy loading" is that the method getPart() returns immediately with the part, and all loading is done in background. To 
know when the part is ready, just add a callback with the whenReady method:

	var partSpec = this.getPart(partName);
	partSpec.whenReady(function(){});
	
If a part is already in the cache with all suparts loaded, it will immediately fire the "whenReady" callbacks, otherwise they will 
be fired recursively as parts are loaded.
This system will gracefully "merge" all requests for a part and guarantees that if the model needs X unique subparts, then at 
most X asyncronous requests are made to the server.

Model Building
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Rendering
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