API BREAK: fix: typedef 'ssize_t' to 'SSIZE_T' on Windows

API BREAK: fix: typedef 'ssize_t' to 'SSIZE_T' on Windows

In several places, code was using `int` as a drop-in replacement for `ssize_t`
on Windows where it does not exist. This is incorrect on some platforms. E.g. on
x86-64 this will result in `ssize_t` being a 32-bit type instead of a 64-bit
type. This change replaces it with the correct Windows equivalent, `SSIZE_T`.¹

¹ https://docs.microsoft.com/en-us/windows/win32/winprog/windows-data-types?redirectedfrom=MSDN#ssize_t

Gitlab: fixes #1804