do not rely on packed structures

Clang on Windows doesn't support the GCC type of packing. And even though we shouldn't read structures right away in memory. We also cannot rely on -mno-ms-bitfields as some project may require it (VLC on Windows or any project mixing libdvdread and Windows code).

Tested successfully with a disc that crashed before in VLC, used via libdvdnav.

Fixes #19