enforce drawing order

the groups per object type listed in the mappings.yaml file should be rendered in order.

Render order:

1. polygons
2. line strings
3. points

Within each shape type, the groups should be rendered in order. e.g.,:

roads:
  - name: dangerous
    paint:
      line-color: red
      line-width: 10
    objects:
      - steep 1
      - steep 2
  - name: all roads
    paint:
      line-color: green
      line-width: 5
    objects:
      - level 1
      - steep 1
      - steep 2

In this example, the dangerous roads will render a wide red line and then the all roads will render a narrow green line. This issue will enforce the order of rendering to match the order the groups are listed in the YAML so the green line is overlayed onto the thicker red line.

‼ if the order from the yaml cannot be preserved, a new optional property will need to be added, e.g., order which is a number representing the order a group is rendered within the shape type.