Partial fix for crash in SGPropertyNode::fireValueChanged

Partial fix for crash in SGPropertyNode::fireValueChanged

The effect system used Listeners on property nodes to get the values
for shader uniforms. These listeners get deleted by an osg thread
causing access to freed memory when this happens while the main thread
calls fireValueChanged.

This patch changes the update method to polling for scalar properties.
This isn't 100% threadsafe, too. But at least it does not crash anymore.