According to MinGW headers (which is used for compiling libmpg123) ssize_t type must be 64-bit in size on 64-bit Windows
Moreover it was impossible to build GZDoom with the recent versions of libmpg123 because of wrong type redifinition
Tested MP3 music on Deus Vult II with Release x64 and OpenAL backend
Instead of the previous method where there'd be a filename and offset, and/or a
memory pointer, this uses a class to access resource data regardless of its
underlying form.
