Add a graphics tablet input demo

[Calinou]

This adds a demo for pen input in Godot, which will also come in handy to test tablet-related PRs like godotengine/godot#76120.

This uses code from godotengine/godot#75903 for drawing (thanks @nlupugla!).

This demo was developed on 4.3, so it can be cherry-picked.

Preview

TODO

• Fix width curve according to pen pressure. Right now, it changes the entire line's width, but it should keep the history of the pressure over the line's length and apply it correctly. I have code in place for that but it not's working correctly for some reason. Please help on this if you can 🙂
  • Add a per-point width property to Line2D godot-proposals#9241 could make this easier to implement.

[bruvzg]

Tested on macOS with Wacom tablet and Apple Pencil, some notes:

• tilt registers but do nothing, it's probably should have at least some indication, at least display a tilt vector, or maybe change color.
• extremely long lines start to lag, so it's probably worth splitting the line at some point.

[Calinou]

I fixed the width curve to work as intended.

• tilt registers but do nothing, it's probably should have at least some indication, at least display a tilt vector, or maybe change color.

I added a tilt vector visualization, but I can't test it as my tablet (Wacom Intuos M 2013) doesn't support tilt.

• extremely long lines start to lag, so it's probably worth splitting the line at some point.

Done (it now splits every 1024 points).

[bruvzg]

I added a tilt vector visualization, but I can't test it as my tablet (Wacom Intuos M 2013) doesn't support tilt.

Seems to be working fine.

PXL_20250209_105211520.mp4

[aaronfranke]

I don't have a graphics tablet so I haven't tested it, but from reading the code on GitHub, it looks great!