Fix scrolling in small wxVListBox with tall items.

Fix scrolling in small wxVListBox with tall items.

Scrolling in a small wxVListBox with tall items (i.e. taller than the height
of wxVListBox itself) behaved wrongly: wrong item was being scrolled into view
and Page Up/Down didn't scroll as much as they should.

Fix both of these problems by checking for these corner cases explicitly.

Closes #13454.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69180 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775