Fix the crash on drag&drop of tiger.svgz.

Fix the crash on drag&drop of tiger.svgz (and likely other *.svgz) into an empty inkscape doc. With an automated test.

add a regression "unit"ish test.

todo: cleanup.

fix indents

Refactoring / code cleanup.

See:

• https://en.wikipedia.org/wiki/Code_refactoring

• https://www.refactoring.com/

• https://www.joelonsoftware.com/2002/01/23/rub-a-dub-dub/

Some small optimisations may have slipped in as well.

non absolute path.