Avoid superfluous calls to SetDirtyAsBlocks when dragging window

See: #170

src/gfx.cpp

@@ -1542,7 +1542,7 @@ void DrawDirtyBlocks()
 		RedrawScreenRect(0, 0, _screen.width, _screen.height);
 		Window *w;
 		FOR_ALL_WINDOWS_FROM_BACK(w) {
-			w->flags &= ~(WF_DIRTY | WF_WIDGETS_DIRTY);
+			w->flags &= ~(WF_DIRTY | WF_WIDGETS_DIRTY | WF_DRAG_DIRTIED);
 		}
 		_whole_screen_dirty = false;
 	} else {
@@ -1562,6 +1562,7 @@ void DrawDirtyBlocks()
 
 		Window *w;
 		FOR_ALL_WINDOWS_FROM_BACK(w) {
+			w->flags &= ~WF_DRAG_DIRTIED;
 			if (!MayBeShown(w)) continue;
 
 			if (w->viewport != nullptr) w->viewport->is_drawn = false;

src/window.cpp

@@ -2244,7 +2244,10 @@ static EventState HandleWindowDragging()
 				break;
 			}
 
-			w->SetDirtyAsBlocks();
+			if (!(w->flags & WF_DRAG_DIRTIED)) {
+				w->flags |= WF_DRAG_DIRTIED;
+				w->SetDirtyAsBlocks();
+			}
 
 			int x = _cursor.pos.x + _drag_delta.x;
 			int y = _cursor.pos.y + _drag_delta.y;

src/window_gui.h

@@ -254,6 +254,7 @@ enum WindowFlags : uint16 {
 	WF_CENTERED          = 1 << 10, ///< Window is centered and shall stay centered after ReInit.
 	WF_DIRTY             = 1 << 11, ///< Whole window is dirty, and requires repainting.
 	WF_WIDGETS_DIRTY     = 1 << 12, ///< One or more widgets are dirty, and require repainting.
+	WF_DRAG_DIRTIED      = 1 << 13, ///< The window has already been marked dirty as blocks as part of the current drag operation
 };
 DECLARE_ENUM_AS_BIT_SET(WindowFlags)