visual typesetting: limit event frequency

This change makes Aegisub a lot more responsive on platforms like X11
where mouse events may come in at very high frequencies and/or in bursts.

src/visual_tool.cpp

@@ -170,6 +170,13 @@ VisualTool<FeatureType>::VisualTool(VideoDisplay *parent, agi::Context *context)
 
 template<class FeatureType>
 void VisualTool<FeatureType>::OnMouseEvent(wxMouseEvent &event) {
+	// wx doesn’t throttle for us, updating the video view is
+	// very expensive, and aegisub’s work queue handling is bad,
+	// so limit mouse event rate to ~200 Hz
+	long ts = event.GetTimestamp();
+	if ((ts - lastEvent) < 5) return;
+	lastEvent = ts;
+
 	bool left_click = event.LeftDown();
 	bool left_double = event.LeftDClick();
 	shift_down = event.ShiftDown();

src/visual_tool.h

@@ -92,6 +92,7 @@ protected:
 	bool holding = false; ///< Is a hold currently in progress?
 	AssDialogue *active_line = nullptr; ///< Active dialogue line; nullptr if it is not visible on the current frame
 	bool dragging = false; ///< Is a drag currently in progress?
+	long lastEvent = 0;
 
 	int frame_number; ///< Current frame number