Add support for tablet relative mode

[CheesecakeCG]

It looks like there was some support for this before but it was removed in #85

Relative mode is pretty much a requirement for using drawing tablets for shooter games and some other genres and for me it makes digital sculpting much easier. When drawing or sculpting, it lets you use much more of the tablet's surface in a single stroke.

[YaLTeR]

Uhh? Is this something a compositor is even involved with? #85 is not really related.

Maybe you need https://opentabletdriver.net? If you don't map the tablet to any output in the niri config, it will map itself as OpenTabletDriver expects, so you should be able to use the OpenTabletDriver options.

[CheesecakeCG]

I think so, Hyprland and swaywm have config options for this.

I think I misread that issue when I skimmed over it, I'd vaguely remembered relative mode working (or more, didn't noticed it didn't default to absolute mode), but I'm probably misremembering things, my bad.

I've been using the digimend kernel drivers, I'll give OpenTabletDriver a try and see how it goes.

Edit: OpenTabletDriver works with this tablet and switching to relative mode works, but there's no pressure sensitivity in relative mode and no mouse button support in "Artist Mode", and their docs say to switch to Artist Mode to get pressure sensitivity and to switch to Relative or Absolute mode to get mouse button input...

[YaLTeR]

Need to check what this sway option does in the code