(#373): Render gradients with userSpaceOnUse even for an empty bounding box

In the test, a horizontal or vertical line was not being styled with a
gradient, because those lines have an empty bounding box.  However,
the gradient in question had gradientUnits="userSpaceOnUse", *not*
"objectBoundingBox".  So, we don't need the object's bounding box at
all, and the lines can use the gradient in the current coordinate
system.

Fixes GNOME/librsvg#373
3 jobs for <span class="ref-name">gradient-userspaceonuse</span> in 8 minutes and 6 seconds (queued for 7 minutes and 48 seconds)
Status Job ID Name Coverage
  Test
passed #140940
fedora test

00:07:41

 
  Lint
manual #140942
allowed to fail manual
clippy
passed #140941
rustfmt

00:00:24