fix #1000: film grain SEI encoder use wrong bits to signal num_model_values_minus1

The syntax element num_model_values_minus1 should be signaled with 3 bits according to spec. The encoder SEI function xWriteSEIFilmGrainCharacteristics() mistakenly uses 8 bits. The decoder parsing is correct.

changed title from fix film grain sei encoder to fix #1000: film grain SEI encoder use wrong bits to signal num_model_values_minus1

merged

mentioned in commit f5d838df