Link underline should be the same as FG color

[Tyriar]

Regression introduced in #938

[peacefullatom]

I'd like to grab this :)

[Tyriar]

@peacefullatom great! 😃

The link underlines are rendered from this file: https://github.com/xtermjs/xterm.js/blob/master/src/renderer/LinkRenderLayer.ts

You can check this code for how to get the foreground color:

xterm.js/src/renderer/TextRenderLayer.ts

Line 139 in 0e45909

let fg = (attr >> 9) & 0x1ff;

[peacefullatom]

@Tyriar, thank you for hints, they were helpful!

This comment by @mofux was very informative as well.

If there is anything I can add/improve just tell me.

Cheers! :)