(#335): Return an error when the toplevel element is not <svg>

We add a simple validation function for the root of the tree.

GNOME/librsvg#335
3 jobs from non-svg-toplevel in 8 minutes and 28 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #103479
fedora test

08:05

 
  Lint
manual #103481
allowed to fail manual
clippy
passed #103480
rustfmt

00:23