GUI: prevent lag when drawing on wave editor

src/gui/gui.cpp

@@ -2419,7 +2419,7 @@ void FurnaceGUI::processDrags(int dragX, int dragY) {
       if (y>waveDragMax) y=waveDragMax;
       if (y<waveDragMin) y=waveDragMin;
       waveDragTarget[x]=y;
-      e->notifyWaveChange(curWave);
+      notifyWaveChange=true;
       MARK_MODIFIED;
     }
   }
@@ -3746,6 +3746,11 @@ bool FurnaceGUI::loop() {
       midiLock.unlock();
     }
 
+    if (notifyWaveChange) {
+      notifyWaveChange=false;
+      e->notifyWaveChange(curWave);
+    }
+
     eventTimeEnd=SDL_GetPerformanceCounter();
 
     if (SDL_GetWindowFlags(sdlWin)&SDL_WINDOW_MINIMIZED) {
@@ -6718,6 +6723,7 @@ FurnaceGUI::FurnaceGUI():
   preserveChanPos(false),
   wantScrollList(false),
   noteInputPoly(true),
+  notifyWaveChange(false),
   displayPendingIns(false),
   pendingInsSingle(false),
   displayPendingRawSample(false),