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

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" }
 });