Own brush workers and thin preview/platform seams

src/platform_linux/linux_platform_services.h

@@ -1,7 +1,11 @@
 #pragma once
 
+#include <functional>
+#include <string>
+
 namespace pp::platform::linux {
 
+void set_fps_title_callback(std::function<void(std::string)> callback);
 void report_rendered_frames(int frames);
 
 }