[core] Add 'auto' enum value to symbol-z-order layout property

include/mbgl/style/types.hpp

@@ -70,6 +70,7 @@ enum class SymbolPlacementType : uint8_t {
 };
     
 enum class SymbolZOrderType : uint8_t {
+    Auto,
     ViewportY,
     Source
 };

scripts/style-spec.js

@@ -2,5 +2,3 @@ var spec = module.exports = require('../mapbox-gl-js/src/style-spec/reference/v8
 
 // Make temporary modifications here when Native doesn't have all features that JS has.
 delete spec.layout_symbol['symbol-sort-key'];
-delete spec.layout_symbol['symbol-z-order'].values['auto'];
-spec.layout_symbol['symbol-z-order'].default = 'viewport-y';

src/mbgl/style/layers/symbol_layer_properties.hpp

@@ -29,7 +29,7 @@ struct SymbolAvoidEdges : LayoutProperty<bool> {
 
 struct SymbolZOrder : LayoutProperty<SymbolZOrderType> {
     static constexpr const char *name() { return "symbol-z-order"; }
-    static SymbolZOrderType defaultValue() { return SymbolZOrderType::ViewportY; }
+    static SymbolZOrderType defaultValue() { return SymbolZOrderType::Auto; }
 };
 
 struct IconAllowOverlap : LayoutProperty<bool> {

src/mbgl/style/types.cpp

@@ -78,6 +78,7 @@ MBGL_DEFINE_ENUM(SymbolAnchorType, {
 });
 
 MBGL_DEFINE_ENUM(SymbolZOrderType, {
+    { SymbolZOrderType::Auto, "auto" },
     { SymbolZOrderType::ViewportY, "viewport-y" },
     { SymbolZOrderType::Source, "source" }
 });