MFPaint: Preview shapes

Super stretch goal / 2024 stuff:
When blinking a corner of a rectangle or center of a circle or something, I propose a setting to blink the entire proposed shape.

According to @dylwhich:
>all the drawing functions would need to be modified to be able to keep track of every pixel they draw and what color it was before for that to work

@Brycey92:
> couldn't you just draw the extra pixels after drawing the existing canvas, then turn off the extra draws for the next fraction of a second, etc?

@dylwhich:
> no, not really. the canvas doesn't exist outside of the framebuffer, so anything that needs to be temporarily drawn over top of it has to keep track of the canvas pixels that are underneath in order to be "un-drawn," and I don't think the current system for doing that would work well at all for something like a max radius filled circle