Lots of rendering tests and fixes

This MR adds 15 new compare-render tests that fell out of the work on !6588 (merged) so I decided to add them - and the fixes to existing rendering code - in a separate MR.

That MR is this one.

I also changed the behavior of the compare-render test a bit so that it's now easier to skip certain tests on certain renderers and so that the --mask etc options are harder to break, both in the correctness sense and in not timeouting, because they can take quite a bit longer.