prores: use natural integer type for the codebook index

prores: use natural integer type for the codebook index

The operations that use it require it to be promoted to a larger (natural)
type and thus perform sign extension on it.

While an optimal compiler may account for this, gcc 4.6 (for x86 Windows)
fails. Using the natural integer type provides a 2% speedup for Win64
and 1% for Win32.

Signed-off-by: Diego Biurrun <diego@biurrun.de>