Terminal graphics value types shared by Rio's VT core and renderer.
This crate holds the plain data types produced by the terminal's image
protocols (sixel, kitty, iTerm2) and the glyph protocol, with no
rendering, GPU, or windowing dependencies. It is the leaf both
rio-vt (the safe terminal core) and sugarloaf (the renderer)
depend on, so the core no longer has to pull the render stack just to
model an image.