21.3.3 Packed Record Layout

pack bits (!= NULL bits!): One bit per packable column:
              FIELD_BLOB: Set if blob is empty. No blob data in record.
              FIELD_SKIP_ZERO: Set if all bytes zero, no data in record.
              FIELD_SKIP_PRESPACE: Set if some blanks stripped in record.
              => The "pack bits" are rounded up to the next byte boundary.
Record contents from the in-memory representation.
Each field is copied verbatim unless packed according
to the "pack bits" paragraph.