sync lib_msvc with cpython

Issue 20160 in cpython relating to ctypes callbacks, large structs, and msvc was fixed on cpython, here is the changeset for python2.7, the pr for python 3 includes alot of whitespace changes as well. It seems we should test/fix this for cffi as well, and/or sync libffi_msvc with cpython HEAD

Added the test in 7b305f61d0e0. Will check if it fails or works on Win64.

It seems to work out of the box. Maybe some change I already did in the past?

Right, it's because _cffi_backend.c already makes copies of structs around calls, both for ffi_call() and in the callback logic.