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/ViewerCell.java
* Gets the font of the cell.
	 * 
	 * @return the font of the cell or <code>null</code> for the default font
	 * 
	 * @since 3.4
	 */
	public Font getFont() {
		return row.getFont(columnIndex);
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see java.lang.Object#hashCode()
	 */
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + columnIndex;
		result = prime * result + ((row == null) ? 0 : row.hashCode());
		return result;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see java.lang.Object#equals(java.lang.Object)
	 */
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)