Fixed termbox2 demo build, added scroll functionality

[Godje]

Fixing the demo

• Demo would not compile due to failure to check out the commit tags (they might have been squashed away or pruned).
• Demo would crash instantly due to a "duplicate ID" issue. I statically assigned an ID to the component that was causing this trouble (termbox_settings).

Added horizontal scroll functionality

The termbox2 renderer and the demo supported scroll functionality previously, but they were never shown in the demo. I added horizontal scrolling for the user to be able to see the rest of the palette test window through .clip exactly as it is done in the documentation.

Things that will stay "broken"

Due to how string slices are rendered, horizontal scrolling will trim the string in the wrong direction. It seems that the only way to mitigate this without touching the renderer is to compute strings with the proper trimmed length, but computed strings would require allocating them outside of the layout build. Implementation of this would bloat the demo code, so I kept this out.

[nicbarker]

Thanks for fixing this one up! I just merged another commit that fixed the base issue that was causing the duplicate ID, but we can leave in the explicit ID cause I think it's a good ID to use explicit IDs where possible anyway 🙂

[Grazfather]

I guess this sort of obviates my PR #540.

[Godje]

@Grazfather yeah. I can technically close this PR and just leave my #525 open with the horizontal text culling fix. Either way, whichever gets merged will fix the first demo build.

[Grazfather]

Cool. I don't care either way, I just wanted the example to build, and I stole your code from #524, and missed that you had also pushed fixes for demo.

[nicbarker]

Apologies for the delay in getting this merged, thanks!