Make color boundary movement more responsive

[hxlnt]

Moving the color boundary is slow (around 2 seconds) because each movement requires changing the style of 400-800 divs, and 202 of these are redrawing the boundary itself (a CSS border added to 102 divs and removed from 102 divs).

There are a lot of ways to improve the sluggishness; one easy fix could be making the boundary its own div that floats on top of or underneath the Astropaint screen.

Some day, it might make sense to move to HTML5 canvas, but having each pixel be a div has benefits as well (namely, easily recoloring by div class), and it'll be a lot of work to refactor just to improve the speed on this one feature. Open to suggestions and PRs!