Slash brush

[spaghetti22]

Add a new "slash" brush with configurable angle, mimicking similar functionality in other painting programs such as XPaint, KolourPaint, old versions of MS Paint.

This is the first paint brush with a custom cursor so that the user can preview the angle and width of the slash, so this PR also implements a framework for individual paint brushes to have different cursors.

[cameronwhite]

The brush is very nice to use 👍

[cameronwhite]

I've added the Matrix3x2D changes that you can try out here, and just had a couple minor suggestions after re-reading the code with fresh eyes :)
Looks good to merge after this!

[cameronwhite]

Thanks!

[pedropaulosuzuki]

The brush is indeed very nice to use! The only thing bothering me a bit is that the angle is not exactly the angle used for the brush. If we type 0deg, we still get some rotation. Same for 90deg. 45deg also looks a bit like 60deg or something.

Screencast.from.04-03-2026.23.48.02.webm

I understand the rotation adds to the style, but as we have an angle parameter, it's a bit weird that the angle doesn't match.

[spaghetti22]

@pedropaulosuzuki Hi, thanks for the feedback; this isn't how it behaves for me. Does it behave that way for anyone else reading this? I may have to investigate why this happens for you; will try to find time to upload a screencast of what it looks like for me later today.

[spaghetti22]

@pedropaulosuzuki Attached is a video of what it looks like on my (Debian testing, KDE Plasma, Wayland) machine where I developed it. I do not know why it is not working correctly for you; is there anything about your environment that I may need to know about?

Bildschirmaufnahme_20260305_195207.webm

[cameronwhite]

I wonder if it's something to do with antialiasing, e.g. if the coordinates are at fractional values? Doing just a single-click makes it easier to see, but I think I see some extra rotation happening in your video too at the start of the stroke

One other thing I noticed while testing this out is that it seems like we lost the behaviour of the brush cursor changing size as you zoom in and out.