This is the pattern that was there before:

  foo.some_len = _rsvg_css_parse_length (str);   /* at creation time */

  ...

  double real_len = _rsvg_css_normalize_length (&foo.some_len, ctx, LENGTH_DIR_HORIZONTAL); /* at rendering time */

This led to a few inconsistencies in the way directions got used -
sometimes cut&paste errors could creep in and a vertical direction
would be used for a horizontal length, for example.

This change makes the pattern be

  foo.some_len = _rsvg_css_parse_length (str, LENGTH_DIR_HORIZONTAL);   /* at creation time */

  ...

  double real_len = _rsvg_css_normalize_length (&foo.some_len, ctx); /* at rendering time */

I.e. lengths get their direction-to-be-resolved-as at creation time,
during parse_length(); this gets stored in a new field length.dir.
Later, normalize_length() knows which direction to use.