You can load custom icons through icon dialog, so support non-square icons just like Kicker does. Code mostly from Kicker.

• Size is sane in both vertical and horizontal panel
• Square icons (and the default one) are still rounded to icon size
• Verified with kolourpaint that square icon looks identical with and without this patch (I still fear there might be a "icon washed out" or "frizzled" regression somehow caused by this on low dpi screens (but who doesn't have a 8K monitor these days)
• Changing icon updates size at runtime
• Resizing panel updates size immediately