Introduce computeOffsets() and reordersBuffers() to BufferGeometry.

computeOffset - Reorder the index buffer to fit in draw offsets of a certain number. (65k by default)
reorderBuffers - Reorder all vertex attributes and grow the vertex stack according to a new size and index mapping.
CTMLoader now automatically uses BufferGeometry's computeOffsets() when useBuffers is active on CTM file load.