Function paint_over_improves 

pub fn paint_over_improves(
    settings: &RustCaptureSettings,
    encoder: EncoderQuality,
) -> bool

Whether a cleanup improves the picture at all: at a constant quality where the paint-over quality is finer than the session’s (the cleanup then moves the session’s quality, see decide_constant_quality); at a constant rate where it is finer than the quality index the rate control last coded at, or the encoder does not say. A rate control that has already refined a still screen past it (a constant rate refines every frame it sends) is left alone: holding the paint-over quantizer there would coarsen the picture. A constant-rate encoder that holds no quantizer is cleaned up by the rate control’s own frames, a refresh and its burst, which it refines; a key frame there would come out at the rate control’s quantizer, which a small buffer starves.