Fix ScrollableThumbnail offset calculation (#792)

Fixes #791.

Author: Felix-Schmid

org.eclipse.draw2d/src/org/eclipse/draw2d/parts/ScrollableThumbnail.java

@@ -66,8 +66,7 @@ public void mousePressed(MouseEvent me) {
 			Dimension selectorCenter = selector.getBounds().getSize().scale(0.5f);
 			Point scrollPoint = me.getLocation().getTranslated(getLocation().getNegated())
 					.translate(selectorCenter.negate()).scale(1.0f / getViewportScaleX(), 1.0f / getViewportScaleY())
-					.translate(viewport.getHorizontalRangeModel().getMinimum(),
-							viewport.getVerticalRangeModel().getMinimum());
+					.translate(getSourceRectangle().getTopLeft());
 			viewport.setViewLocation(scrollPoint);
 			syncher.mousePressed(me);
 			dragTransfer = true;