Function bounding_rect 

pub fn bounding_rect(
    vs: &mut dyn VertexSource,
    path_ids: &[u32],
    start: usize,
    num: usize,
) -> Option<RectD>

Compute the bounding rectangle across multiple paths from a vertex source.

Iterates paths from start to start + num - 1, rewinding each by its path ID (obtained from path_ids), and returns the combined bounding box. Returns None if no vertices are found.

Port of C++ agg::bounding_rect.