Instead of calculating a density that we hope results in the correct
size, we can specify what size we want. This is more robust and easier
to understand. This also allows us to simplify the Makefile quite a bit.
Note that Fedora's packaging of ImageMagick has a bug here:
https://bugzilla.redhat.com/show_bug.cgi?id=2140018
All of the icons are square, only providing the size in one direction is
enough. This change lets us avoid some unnecessary complexity in the
Makefile.