Change from int32_t to size_t, because
    - Length is a measurement of unsigned "size".
    - In general, length values are often used in subsequent calculation
      of memory size which is passed to functions like malloc, whose
      input is size_t not int.
    - The cast from ptrdiff_t to int32_t in wcore_attrib_list_length
      just looked wrong. A cast to size_t makes more sense.

As precedent, strlen() also returns size_t.

Signed-off-by: Chad Versace <chad.versace@intel.com>
Tested-by: Emil Velikov <emil.l.velikov@gmail.com> (msvc/wgl)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>