map is drawn twice on toggling a layer's visibility

[smizuno]

The map is drawn twice on toggling a layer's visibility.

show the problem:

1. load two layers (can be the same; linestrings with lots of points helps to see the problem)
   
2. turn on incremental drawing (this is to make the drawing more noticeable)
   
3. turn off render caching (to be more noticeable)
   
4. as you toggle layer visibility off and on observe the map being drawn twice on each toggle. Also, the wait cursor is put up twice.

The incremental drawing and render caching don't affect the problem.

This was noticed on Windows, but I believe all platforms are affected.