Function wrap_segments 

pub fn wrap_segments(col_count: u32, wrap_width: u32) -> u32

Soft-wrap (W.2, A2): number of display rows a row of col_count columns occupies when wrapped at wrap_width columns. Floored at 1 (an empty row still occupies one display row) and at 1 when wrap_width == 0 (wrapping off). Shared by the host scroll model and both renderers so segment arithmetic is defined in exactly one place.