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.