Comparative tests against other renderers

[DJMcNab]

To ensure that our pipeline is reaching results as expected, it would be good to compare our snapshot tests against existing rendering systems. These could include:

• skia
• tiny-skia
• webrender
• cocoa
• Browser rendering of SVGs

Some of this could be tested by using Piet. The existing tests for Piet can be found in https://github.com/linebender/piet-snapshots, which we could bring up as Vello tests.

This should all happen in CI, as these tests are not useful if they can only be run locally.