Fix border background, list icons, background color of body not working

[KaruroChori]

Closes #3 .
It also solves most of the comments left around as TODOs in code.

PS. Border calculations are (and were) a bit off because of the version of litehtml being used not supporting float pixel position but integers. So no 0.5px borders.
It was fixed litehtml/litehtml#415 but it is not yet in a full release.
Also, the public interface of the library has shifted a lot, so it would be a considerable rework to operate with that.

Aside from that, they are not pixel perfect still:

• there are two paths in code, one for elements with all borders set equal, and one for split borders. They have half a pixel difference in rendering position which should be investigated further... if there was enough time :)
• border radius currently only working for uniform borders.
• in the diagonals between borders there are some light artifacts, they could be removed by overlapping a bit more those quads I guess..

Final disclosure, unlike my other two PRs this one was partially llm-assisted; while I manually tested and fixed some more things on my own, if you are against just feel free to reject it.

[BigJk]

Sorry for the late reply. It seems I missed the notifications for this repo! I will check this out as soon as I can. Thank you for your time! 😄

[BigJk]

I tested it. Works like a charm and is a nice improvement over my hacky code! Thanks for your time and I don't mind that it's LLM-Assisted.

The artifacts and shortcomings could be addressed in the future, but even with them there is already a lot of cool stuff that can be done with it that weren't easy todo in ImGui layout-wise before. At least for my use-cases this works well enough.