Allow shrinking the array with wxArray::resize().

Allow shrinking the array with wxArray::resize().

This method did nothing if its argument was less than the current array size,
as it was just a synonym for SetSize() which was documented to behave like
this, but this was inconsistent with std::vector and wxVector resize() which
does shrink the array, so change wxArray version to shrink it too.

Closes #15195.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73986 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775