Fix cue render order for ass/ssa. - #2137

@icbaker Could you take a look? Thanks!

@icbaker hello, please make a review for the new commit, and hope this pr can merge to 1.6.0 final release.

Please can you add some tests for this?

I suggest:

1. Some parsing tests in SsaParserTest (including negative & unparseable layer values) - assert that the right value ends up in Cue.zIndex
2. An end-to-end test in SubtitlePlaybackTest? This will cover both your parsing code and the re-ordering in the CueGroup constructor.

For (2) I suggest you craft an SSA test file that has cues overlapping in time with different layer values, to demonstrate why the re-ordering has to be done 'outside' SsaParser.

I add parseLayer and parseInvalidLayer in SsaParserTest to cover the layer value in ssa.

How can I add a test in SubtitlePlaybackTest? I have no idea.

Thanks for adding the tests to SsaParserTest, I've added SsaPlaybackTest to cover the layer-based re-ordering inside CueGroup.

I'm going to send this for internal review now. You may see some more commits being added as I make changes in response to review feedback. Please refrain from pushing any more substantive changes as it will complicate the internal review - thanks!

I'm going to send this for internal review now. You may see some more commits being added as I make changes in response to review feedback. Please refrain from pushing any more substantive changes as it will complicate the internal review - thanks!

Thanks a lot for your help. 🫶