dump_video: fix yuv420p10 output
dump_video: fix yuv420p10 output

xiph/daala,ycho/daala,luctrudeau/daala,luctrudeau/daala,ycho/daala,ycho/daala,xiph/daala,xiph/daala,ycho/daala,xiph/daala,xiph/daala,luctrudeau/daala,luctrudeau/daala,luctrudeau/daala,ycho/daala