Avoid pointer conversions to unsigned long in the samples.

This results in errors when using g++ Win64 builds as sizeof(long) <
sizeof(void*) in Win64.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75100 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775