[Feature] WebGL Renderer

I'm looking at supporting shader effects similar to native Ghostty's custom-shaders config, and a WebGL renderer seems like the cleanest path. Is this something you've discussed or have on the roadmap?

I'd be happy to work on a first pass. My use case has narrower Unicode needs than `CanvasRenderer` (no ligatures / emoji / complex shaping), so I'd probably build a bitmap glyph atlas via Canvas 2D ([looking at `@xterm/addon-webgl`](https://github.com/xtermjs/xterm.js/blob/master/addons/addon-webgl/src/TextureAtlas.ts)). I'm expecting a real jump in complexity over canvas, but it seems like a pretty interesting project.

I'd be happy to ship it as a separate package/addon; just trying to gauge if this is actually worth digging into. My other option is to just use the canvas as a texture, accept the performance hit, and call it a day.