[core] send signal to RenderCustomLayer when gl context has been lost

- Prevents cleaning up of GL resources that no longer exist, resulting in a crash

include/mbgl/renderer/renderer.hpp

@@ -28,6 +28,8 @@ public:
              const optional<std::string> programCacheDir = {});
     ~Renderer();
 
+    void markContextLost();
+
     void setObserver(RendererObserver*);
 
     void render(const UpdateParameters&);