Fix for L_PREFER_CANVAS/CircleMarker bug

There is a problem with CircleMarkers when L_PREFER_CANVAS is truthy. If a CircleMarker is added to the map, redraw requests are continually made. It seems that the moveend handler calls _updatePath which calls _updateStyle which calls CircleMarker.setRadius which calls redraw which results in a moveend event, repeating the process.

Here's a JSFiddle that demonstrates the problem:
http://jsfiddle.net/cartant/NSJWa/
It'll write "moveend" to the console again and again and again.

The simplest solution seemed to be the addition of a noRedraw parameter. (I noticed there are functions elsewhere in Leaflet that use a similar parameter.)