Add an overlay to the VML canvas as well so that mouse events works better.
Add an overlay to the VML canvas as well so that mouse events works better.

git-svn-id: b84842894e58b734886c7d9dc308b48641e25c76@71 fd59bba8-8519-0410-9389-e36bf2bdbbd6

GerHobbelt/excanvas,thinkerchan/excanvas,huip/excanvas,thinkerchan/excanvas,huip/excanvas