libvlc: use size_t instead of optional uintptr_t in libvlc

uintptr_t is an optional type in C11 and C++11. For a public API like libvlc it's better to rely on the types found on all C and C++ compilers.

size_t and uintptr_t should be the same on all platforms we support. Maybe we don't need the static_assert once the CI's pass. It would be a silent type change.

Host app will either generate a new error or warning and need to be updated or not and will continue to work as before because they are the same type underneath.

libvlcpp will need to be updated accordingly.