Handle chroma position in 4:2:0 Y4M output

Currently, the 8-bit color space written to Y4M with 4:2:0 sampling is always 420jpeg, but some Y4M decoders will incorrectly interpolate chroma in files generated from streams where chroma_sample_position was set to CSP_VERTICAL or CSP_COLOCATED. In these cases I believe the correct Y4M color spaces are 420mpeg2 and 420paldv respectively.

Reference: FFmpeg