reset depth stencil state for render pass (#3230)

include/mbgl/mtl/render_pass.hpp

@@ -42,6 +42,13 @@ public:
 
     void endEncoding();
 
+    /// Resets the states and bindings of the render pass to deal
+    /// with situations where they are changed by calling the
+    /// underlying rendering API directly. Currently this happens
+    /// when a user changes one of these states or bindings
+    /// in a custom layer
+    void resetState();
+
     void addDebugSignpost(const char* name) override;
 
     void bindVertex(const BufferResource&, std::size_t offset, std::size_t index, std::size_t size = 0);