Added some rules to GtkCellAreaBox for rendering the last cell.

Added some rules to GtkCellAreaBox for rendering the last cell.

 - When we reach a cell that is out of the render area, break out
   of the loop (for columns user resized too small)
 - CLAMP the size of the last renderer to fit into the area
   (so that renderers get a chance to ellipsize when rendered
   with a space less than allocation, same reason as above).
 - Hand out remaining space in the render area to the last cell,
   this is for shallow rows in the expand column which may recieve
   more than the allocated width.