Fix transparent foreground color

[Dennnnny]

fix: #4408

Description:

I found this transparent foreground color will affect some text when the foreground is background
And inside the TextureAtlas, there are already exist some check for background transparent
So I put another check foreground transparent

Hope this make scenes

Any feedbacks is welcome, thanks

---

Screenshot:

For local test, I change the xtermjsTheme foreground color with transparent

// demo/client.ts
const xtermjsTheme = {
  foreground: '#F8F8F8CC'
}

And the when no doing anything, it looks like:

After change:

[jerch]

Have not looked at the PR code, just FYI:
We have no good idea, what transparency on FG should mean or be used for in a terminal. I tried to address this several times with other terminal devs, esp. because SGR has a dedicated transparent flag (which we dont support yet), but the response on that was more like "Idk, have not yet dealt in depth with it".

So until we resolve the issue, what transparency in FG can be used for and find a real good use case, I like @Tyriar 's idea to completely strip any transparency from FG (e.g. introduced by FG/BG colors swaps), maybe with an additional lumi correction to stay on sane ground.

[Tyriar]

Thanks for investigating this 👍

[Dennnnny]

thanks for all the help and suggestions.
it's my honor to contribute ❤️