Add PNG gray render option

src/GLcvApp.cpp

@@ -80,7 +80,7 @@ GLcvApp::GLcvApp(GLFWwindow *window, AppConfig &config) :
 	gui->addButton("Screenshot", [this]() { OnScreenshot(); });
 	gui->addVariable<ScreenshotFormat>("Format",
 		[&config](const ScreenshotFormat &fmt) { config.SetRenderFormat(fmt); },
-		[&config]() { return config.GetRenderFormat(); })->setItems({"PNG", "TIFF", "EXR"});
+		[&config]() { return config.GetRenderFormat(); })->setItems({"PNG", "TIFF", "EXR", "PNG (Gray)"});
 	gui->addButton("Reload", [this]() { OnReload(); });
 
 	screen.setVisible(true);