Enable symbol-sort-key when text/icons on a layer have opposing overlap rules

[chloekraw]

When icon-allow-overlap: true and text-allow-overlap: false or vice-versa for a symbol layer, there is no way to sort icons + text consistently with symbol-sort-key.

Sort key determines draw order in the renderer. Features with lower sort keys are drawn and placed first.

• When overlap is enabled, features with higher sort keys can be drawn and placed over previous features, so features with higher sort keys have higher priority.
• When overlap is disabled, features with higher sort keys cannot be placed over previous features because of the way collision detection works. Therefore, during placement, features with lower sort keys have higher priority.

This behavior has always existed in the renderer and it won't be straightforward to change.

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[ryanbaumann]

As a developer, I want to create a rank value for features in a symbol layer to control where they are drawn and placed. The challenging part is when I have to rank features differently and/or inversely when I toggle collision detection between symbol features on/off.

A simple solution for a developer would be to have an "always high values" or "always low values" are prioritized using symbol-sort-key, and not needing to change the ascending vs. descending order when I toggle collision detection on or off.