vertexcodec: Move first vertex to the end of the stream
vertexcodec: Move first vertex to the end of the stream

Additionally pad the vertex to 32 bytes. This makes sure that we have 32
bytes at the end of the stream which simplifies bounds checks, and does
this at the minimal cost to the encoded size.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer