indexcodec: Switch nibbles in the first triangle byte
indexcodec: Switch nibbles in the first triangle byte

This does not change the encoding efficiency, but makes the hex output
of the codec slightly easier to read - logically, it makes sense that we
first encode the edge index (or 0xf to signify a 3-vertex triangle),
followed by the vertex index - now the nibble order matches this.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer