Webgl: Support rendering underline attribute

[Tyriar]

Webgl addon added in #1790

[kamalesh0406]

Can you elaborate on what feature you need I cannot understand it 😅

[Tyriar]

Currently echo -e '\x1b[4mhello' will not print hello in underlined like the other renderers:

webgl:

expected:

This will also involve a little shader programming to draw the underlines on top of the background, under the text.

[ryanwarsaw]

@Tyriar another issue I've noticed is that font ligatures don't appear to work in the new renderer. Are these two issues similar? (in the sense they're not implemented?) I'm new to the project, would love to contribute - not sure where the best place to start would be!

[Tyriar]

@ryanwarsaw ligatures are broken with all renderers at the moment, We need to tweak the addon in #2318

Once that's done, for WebGL we would need to handle these:

xterm.js/src/renderer/Renderer.ts

Lines 201 to 207 in 5a9e6fe

	public registerCharacterJoiner(handler: CharacterJoinerHandler): number {
	return this._characterJoinerRegistry.registerCharacterJoiner(handler);
	}
	public deregisterCharacterJoiner(joinerId: number): boolean {
	return this._characterJoinerRegistry.deregisterCharacterJoiner(joinerId);
	}

In the WebGL renderer:

xterm.js/addons/xterm-addon-webgl/src/WebglRenderer.ts

Lines 225 to 231 in 5a9e6fe

	public registerCharacterJoiner(handler: (text: string) => [number, number][]): number {
	return -1;
	}
	public deregisterCharacterJoiner(joinerId: number): boolean {
	return false;
	}

Right now they do nothing but they need to register/deregister and then be used when drawing the characters together and treat them as a simple glyph. For example if the addon says ">=" is a ligature, it should draw ">=" together in a single glyph and save it to the texture atlas like that, you might be able to leverage WelglChatAtlas.getRasterizedGlyphCombinedChar to do that.

You might be able to hack in the ligatures addon to get it to work of bring it into this repo (from https://github.com/xtermjs/xterm-addon-ligatures) to get started on this, it should be relatively easy to do #2318 if you want to give that a shot.