Function convert_row 

pub fn convert_row(plan: &ConvertPlan, src: &[u8], dst: &mut [u8], width: u32)

Convert one row of width pixels using a pre-computed plan.

src and dst must be sized for width pixels in their respective formats. For multi-step plans, an internal scratch buffer is allocated per call. Prefer RowConverter in hot loops (reuses scratch buffers).