fix: vec of contours size change, use len instead of capacity in dmesh

[PatrickChodowski]

I encountered panicked at /rerecast-0.2.0/src/contours.rs:131:54: index out of bounds: the len is 275 but the index is 275 which I assume is caused by wrong method to expand the vector capacity. Truncate seems to have no effect if we try to set the size larger than it is currently.

[janhohenheim]

Looking at the original, this seems indeed to be the correct way. Thanks! Will merge when CI is green :)

[PatrickChodowski]

Sorry for using the same PR, I can open a new one if needed. I havent realized its the same branch :/

In detail mesh I am encountering substract with overflow error on dmesh.vertices.reserve(vcap - dmesh.vertices.capacity()); since there is a comparison to vertices.len() I figured len() should be used in ```.reserve()`` calculation too :)

[janhohenheim]

Took a little while to think this through, but yes, this looks correct, as the docs for Vec state that reserve(additional) results in len + additional, not capacity + additional
Thanks for the fix!