(#372): Fix mis-rendering in small arc segments

The code was checking for division by 0.0 a bit too aggressively.  For
small radiuses (say, 0.25 units), this gives intermediate results that
are perfectly (and exactly!) representable within f64, but the code
was checking for 0 too coarsely.

Regenerated the test reference files for Adwaita that got minor
rendering changes because of this.

GNOME/librsvg#372
3 jobs for <span class="ref-name">small-arc-segments</span> in 7 minutes and 31 seconds (queued for 4 minutes and 32 seconds)
Status Job ID Name Coverage
  Test
passed #140012
fedora test

00:07:08

 
  Lint
manual #140014
allowed to fail manual
clippy
passed #140013
rustfmt

00:00:22