fix #5827 unblock labels with overscaled tiles correctly

[ansis]

This fixes #5827 by using overscaledZ instead of canonical.z, letting overscaled tiles unblock labels correctly.

Launch Checklist

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

@jfirebaugh

[jfirebaugh]

Is it feasible to write a render regression test for this?

[ansis]

I think it should be feasible to add a regression test but I'm not sure it's worth it in this case. The changes I'm porting back from -native change the relevant part of CrossTileSymbolIndex entirely and pretty much eliminate this possibility of regressing this.