Canvas: Avoid flicker using externalGraphic

Flicker is best seen dragging a point using a style with externalGraphic.

Although the externalGraphic is in cache by dragging/selecting a point
then momentarily the vector layer shown empty until the point appear again.

Observed using browsers Chrome, FF and Opera but not in IE9.
Merge pull request #1096 from jorix/canvas_flicker

Canvas: Avoid flicker using externalGraphic