Ensure that size in the major direction of box sizer doesn't exceed the total.

Ensure that size in the major direction of box sizer doesn't exceed the total.

After fixing the problem with "growing items by negative proportion" in r56010
(which still was the correct thing to do as it fixed such indefensibly broken
behaviour as shrinking items with larger proportion by more than "smaller"
items when there was not enough space) the items in a box sizer could become
larger than the total space allocated to the sizer resulting in only parts of
them being visible.

Fix this by truncating the items to the (remaining) total size even if this
means making them less than their minimal sizes -- because there is nothing
else we can do when the total space is smaller than the sum of minimal sizes
anyhow.

Closes #10008.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63704 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775