Function apply_transition 

pub fn apply_transition(
    frame_a: &[u8],
    frame_b: &[u8],
    width: u32,
    height: u32,
    progress: f64,
    transition_type: &TransitionType,
    opts: &TransitionOptions,
) -> Vec<u8> 

Composite two RGBA frames during a transition. progress goes from 0.0 (fully frame_a) to 1.0 (fully frame_b).