fix #6548, never group symbol layers by layout

[ansis]

@ChrisLoer this is a possible fix for #6548 and alternative to #6550

Instead of storing CrossTileIDs per-layer this just makes it so that symbol buckets never have more than one layer. The layer grouping is an attempted optimization to reduce bucket creation time but this doesn't really matter for symbol layers because there aren't really very many (any?) production uses for having multiple layers with the same layout properties.

EDIT: at first this seemed like a nicer approach than #6550 but I'm starting to lean back towards #6550. The other pr seems like a cleaner more logical exception, but this approach could possibly avoid some future bugs from layer sharing.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• [N/A] document any changes to public APIs
• [N/A] post benchmark scores
• manually test the debug page

[ChrisLoer]

I was thinking of an approach like this too, but I realized that this approach changes behavior from pre-#5150, while #6550 (I think) restores pre-#5150 behavior.

@jfirebaugh gave an example of something you might want to do that takes advantage of grouping symbol layers by layout: add a text layer, and then add a second with text-translate in order to implement a drop-shadow. Grouping by layout allows the two layers to share collision detection behavior. I don't think that's something we have to support, but I think it's a good argument that, other things being equal, we should avoid changing behavior here.