Fix format strings parsing to understand C99 %zu etc.

Fix format strings parsing to understand C99 %zu etc.

The parser used to understand only 'Z' specifier for size_t/ptrdiff_t,
which is non-standard libc5 extension. C99 defines 'z' for this purpose,
so use that. Compatibility with 'Z' is preserved.

Also support Visual C++'s non-standard 'I' modifier with the same
meaning.

Fixes #12192.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64800 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775