refactor export equirectangular

src/node_panel_brush.cpp

@@ -98,9 +98,9 @@ void NodePanelBrush::init()
                     img.gayscale_alpha();
 
                 auto thumb = img.resize(64, 64).resize_squared(glm::u8vec4(255));
-                thumb.save(path_thumb);
+                thumb.save_png(path_thumb);
                 //auto po2 = img.resize_power2();
-                img.save(path_high);
+                img.save_png(path_high);
 
                 NodeButtonBrush* brush = new NodeButtonBrush;
                 m_container->add_child(brush);