layout property for sorting symbols

[ansis]

We need some property to control whether symbols are sorted so that we can implement mapbox/mapbox-gl-native#988.

With the new labeling this is easy to implement: mapbox/mapbox-gl-js@5bf094f

Do we want to support sorting in any direction?

The property could be a

angle

"symbol-sort": 0 would sort bottom-up and "symbol-sort": 90 would sort left-to-right

enum

"symbol-sort": "y-ascending" or something

boolean

"symbol-sort-y": true

[jfirebaugh]

Is a style property necessary? What if symbols always sort top to bottom relative to the viewport?

[ansis]

Always sorting sounds good to me.

Should sorting only affect draw order and not placement order?