Workaround Opera SVG event dispatch bug

Opera will not dispatch mouse events to an absolutely
positioned SVG element:

http://bl.ocks.org/jfirebaugh/6fbfbd922552bf776c16

We need layers to be absolutely positioned in order to
obtain the correct positioning and z-order simultaneously.
Therefore we must wrap the SVG in a div and absolutely
position that instead.

Fixes #1416.