Review the new sequence header syntax elements for reduced_still_picture_header / single_picture_hdr_flag

The sequence header OBU has a boolean syntax element reduced_still_picture_header that can be set to 1 to omit syntax elements in the sequence and frame headers that are irrelevant to still pictures.

Note: reduced_still_picture_header will be renamed single_picture_hdr_flag. In this issue we will use the original name.

Many syntax elements have been added to the sequence header OBU. We should review the new syntax elements and see which ones can be omitted when reduced_still_picture_header is set to 1.

For example, syntax elements with "motion" or "mv" in their names are likely to be irrelevant to still pictures.