Display real FPS in the 3D editor instead of estimating based on CPU/…

…GPU time

This reverts to the 3.x method for displaying FPS in the View Frame Time panel.
This has two consequences:

- The reported FPS will no longer go past V-Sync or framerate limiters.
- Unfocusing the window will display a low framerate (10 FPS by default),
  since the FPS is actually being limited by the engine's low-processor mode.

Constant redrawing is still forced while the panel is visible, so there's
no risk of having low-processor mode interfere.

editor/plugins/node_3d_editor_plugin.cpp

@@ -2892,8 +2892,8 @@ void Node3DEditorViewport::_notification(int p_what) {
 						frame_time_gradient->get_color_at_offset(
 								Math::remap(gpu_time, 0, 30, 0, 1)));
 
-				const double fps = 1000.0 / gpu_time;
-				fps_label->set_text(vformat(TTR("FPS: %d"), fps));
+				const double fps = Engine::get_singleton()->get_frames_per_second();
+				fps_label->set_text(vformat(TTR("Editor FPS: %d"), fps));
 				// Middle point is at 60 FPS.
 				fps_label->add_theme_color_override(
 						"font_color",