fixed possible deadlock in StyledInstanceMarker

Reseting the marker usually happens outside the paint events, thus it
should not potentially block the painting that occurs in worker threads.
In the deadlock, locks were on StyledInstanceMarker and
AbstractMarker.areas through reset (e.g. main thread) and vice versa from
the (background) painting worker thread.

Change-Id: I1430f9fdbb7a0fd9682fc259c3da0a41bb1ef6dd
Reviewed-on: http://gerrit.esdi-community.eu/3104
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>