Add tests for filters
Currently we only have the filter tests filters-color-02-b.svg
and filters-composite-02-b.svg
. All others are untested
I'm not sure about the strategy here. Should we trivially port one effect to Rust, then add the test files, then fix it - or first add the test files, fix it in C, then port to Rust? I think the first option may be easier/safer/etc.