Draft: utils: Base nominal svg cursor size on svg document size

In cursor themes like Breeze or Breeze Light, the source svg files have cursors with the nominal size 24. But there are also cursor themes like Bibata and all of its derivatives that have nominal size 256, which can pose additional obstacles for adapting the svg cursor format.

This change makes the svg cursor reader base the nominal cursor size on the document size in order to make the svg cursor format adaption process more convenient for cursor themes that are developed with much bigger svg canvas sizes.