Introduce computeOffsets() and reorderBuffers() to BufferGeometry.

computeOffsets() - Reorder the geometry index buffer and attributes so that all indices are referencing a maximum number (defaults to 65k).
reorderBuffers() - Reorder and grow the vertex attributes to adhere to a new index map.

Ref: #3524
CTMLoader can now properly load very large models by using now uses BufferGeometry's computeOffsets.

This ensures that all the index buffer uses only 16-bit addressable vertices per draw offset.
Ref: #3524