Explicitly cast size_t to int to suppress MSVC warnings.

Explicitly cast size_t to int to suppress MSVC warnings.

Implicit size_t-to-int conversions provoke warnings when building 64 bit
version (in which sizeof(size_t) > sizeof(int)) with MSVC, so make them
explicit as the size_t values used here will always fit in int range.

Closes #14113.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70940 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775