ivf: Use int64_t instead of off_t
off_t isn't available in all environments. On Ubuntu 16.04, GCC 5.4, glibc 2.23, off_t is defined by stdio.h normally, but if building with -std=c99 (which dav1d does), it no longer is defined. If fcntl.h is included additionally (which isn't done in dav1d), it still gets defined though. off_t isn't available in MSVC at all.