Add support for viewport-y symbol-z-order when using symbol-sort-key

Nobody has claimed this yet.

## New Feature

I would like to be able to have annotations sorted based on their viewport-y while also having specific annotations shown over top using symbol-sort-key. I would expect annotations with the same symbol-sort-key to be sorted based on their viewport-y and drawn over symbols with a lower symbol-sort-key.

## Why

This would allow "selected"/"active" annotations to be displayed above "unselected"/"inactive" annotations while using viewport-y ordering.

This would allow for behaviour that aligns with how Google Maps orders their markers and would ease the transition to Mapbox.

It would currently be possible to do this using multiple layers, but would quickly get out of hand when using many symbol-sort-keys.

Start by locating the existing symbol-sort-key and viewport-y ordering paths in the iOS map-rendering code, then determine how equal sort keys are handled. Done means annotations are ordered by viewport-y within equal symbol-sort-key groups while higher keys remain above lower ones.

Written by the indexing model from the issue text.