preliminary work: https://github.com/microsoft/terminal/tree/dev/duhowett/background-atlas

I've been thinking a bit about the DX renderer drawing a "background atlas" before it draws any text.

If we separate background drawing into a totally separate phase, we get the following benefits:

• We do not need to break runs of text until their foreground presentation changes (performance)
• We will not break ligatures when the background changes (correctness) (fixes Ligatures do not render under cursor column in Vim #800)
• We can draw stuff behind the text (visuals, correctness) (touches mini-spec Add a Mini-spec for cursorTextColor #6151)

It would look, roughly, like this:

step 1 (empty renderer)

step 2 (drawing background atlas)

step 3 (drawing foreground)

may need a mini-spec