Bug 703692 - Unable to Resize Large Left Dock Area

gimp_container_grid_view_viewport_resized(): set the wrap_box'
required width to just one cell_width, not cell_width * columns. This
way it can shrink also when attached to a toolbox dock. It remains
unclear why it could nicely shrink in all other dock columns.

(cherry picked from commit 6a5e37b6)