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Generate pre-scaled cursor files for those who don't use cursor-shape-v1

It seems that cursor-shape-v1 is "the path forward". However, according to https://wayland.app/protocols/cursor-shape-v1#compositor-support, currently only kwin implements it. So probably we won't see adoption in non-Qt apps very soon. And there are also X11 apps.

breeze!380 (merged) pre-scales the 24px Breeze cursor to global scaling factors up to 300% in 25% steps. It solves many cursor size issues in Wayland, especially in a fractional scale like 250%:

  1. Electron apps fallback to the closest match, so the cursor is slightly smaller, 48px (200%) in Wayland.
  2. GTK3 apps may have some bug in cursor scaling code, so the cursor is huge, like 900% in Wayland.
  3. GTK4 apps have the cursor in correct size. But they probably upscale from the 24px cursor, so it looks a little more blur than the cursor drawn by kwin, which upscales from 48px.

However, this approach has many limitations:

  1. It doesn't work if the user selects other cursor themes.
  2. It doesn't work if the user sets a larger cursor size, e.g. 36px, and a 300% scale. That would require a 108px cursor which we didn't pre-generate.

So I propose:

  1. Create a standalone executable that pre-scale the current cursor theme to the global scale factor.
    1. It should use the same scaling logic used by kwin (I guess it's https://invent.kde.org/plasma/kwin/-/blob/master/src/compositor_wayland.cpp?ref_type=heads#L272).
    2. It should scale the cursor to the current global scale, and to the integer scale for apps that don't support fractional scaling. (And probably also include a 1x version as fallback)
    3. It should generate the cursor theme in $XDG_DATA_HOME/icons. And that new theme will be set as the cursor theme, instead of what user selected.
  2. Whenever the user changes cursor theme, size, or global scale in systemsettings, the above executable is invoked.
  3. Other DE/compositors might use it too.
  4. To produce more crisp cursor images, that tool might optionally support generating cursors from SVGs, instead of upscaling from pixmaps.

@ngraham

Edited by Jin Liu