fix: ImHex using way too much resources still

Now frames are rendered at only 5 FPS if no input events are being processed. Thanks a lot to @Tey. Properly fixes #189 now

plugins/builtin/source/content/footer_items.cpp

@@ -7,7 +7,7 @@ namespace hex::plugin::builtin {
         ContentRegistry::Interface::addFooterItem([] {
             static float framerate = 0;
             if (ImGui::HasSecondPassed()) {
-                framerate = ImGui::GetIO().Framerate;
+                framerate = 1.0F / ImGui::GetIO().DeltaTime;
             }
 
             ImGui::TextUnformatted(hex::format("FPS {0:.2f}", framerate).c_str());