Add compositor to tui framework

[nazmulidris]

Problem

Currently RenderOps and RenderPipline end up generating crossterm commands which are transmitted to stdout.

• There is an inefficiency w/ this approach since ZOrder can't be handled efficiently.
• It is currently handled by fully flushing crossterm commands at a higher ZOrder followed by a lower ZOrder and so on.
• Take an ex of a modal dialog, which requires some inefficient painting to stdout for each ZOrder that has to be rendered.
• To make things worse, crossterm is slow, and when using bg color styled text, or lots of fg_colors (eg: lolcat) it just falls on its face and flickers like crazy and this is unacceptable. The compositor should fix this by only painting diffs.

One solution is to create a compositor that takes the RenderPipeline and translates it into an intermediate representation (IR) which is quite simply a 2d array of PixelChar. A PixelChar is a struct that is defined as:

1. GraphemeSegment
2. style information (from style: fg_color, bg_color)
3. attrib information (from style: bold, italic, etc)

There are as many PixelChars as there are window_size.col * window_size.row.

So the RenderPipeline has to be rasterized into a grid of PixelChars. Then this grid can be painted using whatever output device & library:

1. crossterm
2. termion
3. webgl
4. icedrs

One other thing that changes between these different outputs is the way input is captured. However, all that input can be normalized to a generic InputEvent type that currently exists today.

Notes

Might also need to use a diffing library like imara-diff to calculate the diffs between a sequence of PixelChar grids.

• https://www.reddit.com/r/rust/comments/ydi7xu/announcing_imaradiff_a_reliably_performant/
• https://github.com/pascalkuthe/imara-diff

Here's another rasterizer built atop termion

• https://github.com/nazmulidris/rust-sloth