Sort key not working across different layer types

If you have 2 layers of different types, the sort keys won't be taken into consideration, meaning that fill layers could be rendered on top of a symbol even though they have sort keys that suggest that the symbol should be on top.

Steps to Trigger Behavior

1. Create a symbol layer with a single item, have a sort key of 100
2. Create a fill layer of a single polygon that contains the symbol location, make it have a sort key of 90
3. The fill layer will be above the symbol

Expected Behavior

Sort key will be calculated among all layer types and be global

Actual Behavior

Sort keys only work on same layer types

Start by reproducing the issue with the two layer types and sort keys described in the steps. Trace the layer-ordering path that handles symbol and fill layers, then verify that a symbol with key 100 renders above a fill with key 90 and that same-type ordering remains correct.