Compilation fix to propgrid sample after r74628.

Use wxMax() and wxMin() instead of std::max() and min().

Also work around wrong for loop scoping for VC6.

See #15368.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74642 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775