WIP: Feature/#388 reworked framegraph

Contains #54 #281 #298 #361 #388 #389 #391 #393 #394 Might convert it to a milestone instead.

library/phx/display_system.cpp

@@ -22,8 +22,17 @@
 
 #include "display_system.hpp"
 
+#include <vector>
+
 namespace phx {
-DisplaySystem::DisplaySystem(Engine* engine) : System(engine) {}
-DisplaySystem::~DisplaySystem() {}
+void DisplaySystem::Update(const FrameTimer::TimeInfo&) { tick(); }
 
+std::vector<di::opengl_window*> DisplaySystem::OpenGLWindows() const {
+  std::vector<di::opengl_window*> opengl_windows;
+  for (auto window : windows()) {
+    auto opengl_window = dynamic_cast<di::opengl_window*>(window);
+    if (opengl_window) opengl_windows.push_back(opengl_window);
+  }
+  return opengl_windows;
+}
 }  // namespace phx