Implement alpha-modifier and single-pixel-buffer protocols

[YaLTeR]

Important: it's not enough to just delegate them; niri's rendering helpers will also need to learn about these two. For example, snapshotting in https://github.com/YaLTeR/niri/blob/7bfdf87bf0138d602888fc3167921bc1d029b0ab/src/render_helpers/surface.rs#L56