Replace sort to min_element in SubgraphDepthLocater::getDepth

[dkvash]

SubgraphDepthLocater::getDepth uses heavy std::sort to find minimum DepthSegment, this patch replaces sort to a more appropriate std::min_element.

Origin implementation also relies on a Collections.min instead of heavy sort. ​https://github.com/locationtech/jts/blob/jts-1.16.1/modules/core/src/main/java/org/locationtech/jts/operation/buffer/SubgraphDepthLocater.java#L53

[Daniel Baston <dbaston@…>]

In bd66c5a/git:

Avoid unnecessary sort in SubgraphDepthLocater

Patch by dkvash
Fixes #985

[dbaston]

Thanks!