Fix a tearing issue while dragging a new rect.

Before this commit, dragging a new rect would result in one
of the lines lagging behind, because it is drawn in the middle
of regeneration. After this commit, the rectangle stays rectangular.

For a reason I do not understand, this only fixes Win32; GTK
continues to exhibit the bug, whereas Cocoa has never exhibited it
but the render latency seems to have lowered slightly.

src/draw.cpp

@@ -454,7 +454,7 @@ void GraphicsWindow::HitTestMakeSelection(Point2d mp) {
 
     if(!sel.Equals(&hover)) {
         hover = sel;
-        PaintGraphics();
+        InvalidateGraphics();
     }
 }