Avoid an MSVC warning about conversion to smaller data types
After 8f320d59, MSVC started producing this warning:
[63/123] Compiling C object src/libdav1d.a.p/obu.c.obj
../src/obu.c(708): warning C4244: '=': conversion from 'uint16_t' to 'uint8_t',
possible loss of data