vertexcodec: Use zigzag encoding for deltas
vertexcodec: Use zigzag encoding for deltas

This doesn't make much of a difference for post-deflate size - this
makes sense since deflate cares about the number of unique values, not
really about the values themselves - but this will make it easier to
analyze the data and compress using integer packing.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer