plugins/startupfeedback: Scale icon with output scale

On Wayland, the mouse cursor is scaled by the output scale automatically. Scale the startup feedback icon accordingly.

---

On X I had to change my cursor size to 48px or whatever explicitly. Wayland does that automatically. Didn’t actually test on X… but my bouncing cursor is no longer miniscule.

I know this isn’t a fully proper fix since you can move the cursor between outputs of different scale but it’s simple enough and fixes a real-world usecase of a single screen at least. The animation is also typically short anyway and the next time a startup comes around, the scale is re-evaluated. Rather than spend more engineering effort on this we should probably investigate rewriting it in QML.