The Adwaita icon theme ships spinners in a scalable directory
with MaxSize=32 and Scale=1. One way to make them scale up in
hi-dpi would be to add an @2 directory with MaxSize=32 and Scale=2,
but that directory would also be consulted in non hi-dpi situations
and give us an effective spinner max size of 64.

Instead, treat svg icons implicitly as hi-dpi, and scale them
up to MaxSize * 2 when in hi-dpi.