Bug 267117 [JFace] Provide possibility for subclasses to check if a column is visible in ViewerRow
Currently this is decided by checking with width but this makes Nebula-Grid fail.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ViewerRow.java
* 
	 * @return the path
	 */
	public abstract TreePath getTreePath();
	public abstract Object clone();
	/**
	 * @return the model element
	 */
	public abstract Object getElement();
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result
				+ ((getItem() == null) ? 0 : getItem().hashCode());
		return result;
	}
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		final ViewerRow other = (ViewerRow) obj;
		if (getItem() == null) {
			if (other.getItem() != null)
				return false;
		} else if (!getItem().equals(other.getItem()))
			return false;
		return true;