Function simplify_with_features 

pub fn simplify_with_features(
    mesh: &mut SimpleMesh,
    is_feature: &[bool],
    target_triangles: usize,
)

Simplify mesh using QEM edge collapses, preserving feature vertices (those for which is_feature[v] == true will never be the removed vertex).

Reduces until at most target_triangles remain or no more safe collapses are available.