Buffered scene graph property that allows weighted accumulation; used internally.
			-- binding 
			-- typeName 
			-- valueSize 
		
			Buffer with size [page:PropertyMixer valueSize] * 4. 
			This has the layout: [ incoming | accu0 | accu1 | orig ]
			Interpolators can use .buffer as their .result and the data then goes to 'incoming'.
			'accu0' and 'accu1' are used frame-interleaved for the cumulative result and
			are compared to detect changes. 'orig' stores the original state of the property.
		
Default is *0*.
Default is *0*.
Default is *0*.
			Accumulate data in [page:PropertyMixer.buffer buffer][accuIndex] 'incoming' region into 'accu[i]'.
			If weight is *0* this does nothing.
		
Apply the state of [page:PropertyMixer.buffer buffer] 'accu[i]' to the binding when accus differ.
Remember the state of the bound property and copy it to both accus.
Apply the state previously taken via 'saveOriginalState' to the binding.
[link:https://github.com/mrdoob/three.js/blob/master/src/[path].js src/[path].js]