Color 3D editor FPS label depending on the performance level

This provides easier visual grepping.

Incindentally, this change will force constant editor redrawing whenever
the View FPS option is enabled in the 3D viewport settings. This is
required to get an accurate FPS display in 3.x.
Otherwise, the FPS counter shot all the way up to 145 FPS
whenever nothing was changing on screen (even if actual
performance was much lower).

editor/plugins/spatial_editor_plugin.h

@@ -285,6 +285,7 @@ private:
 
 	VBoxContainer *top_right_vbox;
 	ViewportRotationControl *rotation_control;
+	Gradient *frame_time_gradient;
 	Label *fps_label;
 
 	struct _RayResult {
@@ -487,6 +488,7 @@ public:
 	Camera *get_camera() { return camera; } // return the default camera object.
 
 	SpatialEditorViewport(SpatialEditor *p_spatial_editor, EditorNode *p_editor, int p_index);
+	~SpatialEditorViewport();
 };
 
 class SpatialEditorSelectedItem : public Object {