iconview: gtk_icon_view_compute_n_items_for_size bugfix

iconview: gtk_icon_view_compute_n_items_for_size bugfix

While working on the "iconview: Don't shrink items" patch I noticed that
gtk_icon_view_compute_n_items_for_size modifies the natural and minimum
item sizes it got from gtk_icon_view_get_preferred_item_size when
calculating the max number of items which will fit, but later on it
checks against these sizes when calculating the item_size, and these
checks expect these values to be unmodified.

This patch fixes this by modifying the natural and minimum values in
advance and doing all computations with modified values.

https://bugzilla.gnome.org/show_bug.cgi?id=677809