Implement center on position when using WebGL rendering

Implement a way to center the view on a specific position when using WebGL rendering.
The result is similar to what is shown in the 'zoomPanAPI' demo which uses SVG rendering.
Centering on a node can be done like this:
```
var nodeId = $('#nodeid').val();
if (graph.getNode(nodeId)) {
  var pos = layout.getNodePosition(nodeId);
  renderer.moveTo(pos.x, pos.y);
}
```