Function render_blend 

pub fn render_blend(
    pres: &Presentation,
    old: usize,
    new: usize,
    mix: f64,
    shapes: &[(Shape, bool)],
    text: &[TextMorph],
    paths: &[PathMorph],
    opts: &RenderOpts<'_>,
) -> Image

Render a frame between slides old and new (indices into pres.slides), e.g. of Morph: the backdrop (background, master and layout graphics) of old cross-fading into that of new by mix (0..1), then shapes in order, each resolved against the slide it comes from (true: old). Pairs of shapes in text draw their boxes as usual but their texts morph by words or characters (TextMorph), on top of the later shape of the pair. Shapes listed in paths draw an outline morphing from another geometry into their own (PathMorph). opts.state applies to shapes only.