Wrong format string specifier for size_t argument in demo/overview.c when compiling on newer compilers

I get warnings when compiling some of the demo files in VS2019:

> 1>demo\overview.c(403,33): warning C4477: 'sprintf' : format string '%lu' requires an argument of type 'unsigned long', but variadic argument 1 has type 'size_t'
> 1>demo\overview.c(403,33): message : consider using '%zu' in the format string
> 1>demo\overview.c(415,33): warning C4477: 'sprintf' : format string '%lu' requires an argument of type 'unsigned long', but variadic argument 1 has type 'size_t'
> 1>demo\overview.c(415,33): message : consider using '%zu' in the format string

There's a similar warning in clang as well.

Seems that using the correct format specifier wouldn't be compatible with C89/C90, so this isn't quite as trivial as the fix the warning suggests.

Inspect demo/overview.c at the reported lines 403 and 415, then compare the format-string requirements in VS2019 and clang with the project's C89/C90 compatibility constraint. Done means the size_t warnings are addressed without violating the stated compatibility requirement.

Written by the indexing model from the issue text.