Description of the new feature/enhancement

From old ideas of reusing CustomTextLayout, it will be more efficient to cache the entire layout for further usage. A classic space-time tradeoff.

Proposed technical implementation details (optional)

It's a common practice to cache the glyph layout. On macOS, CoreText will internally keeps a cache about glyphs. Alacritty will prematurely set up a cache which contains the glyphs of common ANSI characters. However because Alacritty does this and rendering on character level, it fails to handle ligature correctly.

We've chosen to render run instead of single character. That gives us ligature. Could we also cache the layout by run? The cache hit rate will of course be worse than caching by character. But I feel it'll still be worth the effort.