simplify: Don't push redundant collapses for manifold edges
simplify: Don't push redundant collapses for manifold edges

We encounter each manifold edge twice (once for each adjoining
triangle). We treat edges like this as bidirectional, evaluating both
collapses each time and pushing the minimum error one - for border edges
this is crucial, but for manifold edges it leads to doing redundant
work.

We now skip manifold edges with i1>i0 since presumably we'll process
them when i0>i1. This significantly improves simplification performance
on large manifold models - 5 LODs for buddha.obj are now generated in
1.0s instead of 1.5s before this change.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer