Test SVG importing

SVG importing has a number of correctness issues. A test suite could help identify issues and reduce chances of regressions.

A possible approach would be to do image comparisons with the following:

• Render directly with resvg
• Import to graphite, render the SVG from the node graph, then render the output with resvg.
• Import to graphite and render with vello

Possible suites of SVG images to use are:

• rsvg-test-suite
• Web Platform Tests
• Inkscape examples
• etc

Note that an automated pixel comparison would be perhaps fragile due to colour spaces and text anti-aliasing and stem thickening and font versions. For now the text can be skipped since Graphite currently doesn't do very well with text importing.