Compute edge error as a sum of errors
Compute edge error as a sum of errors

This seems to give slightly better results than std::max and matches the
paper better; I'm still unclear as to whether we need bidirectional or
unidirectional error (it seems intuitively that we should use
unidirectional error but the results don't confirm that).

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer