Disallow offset-position-v as a one component position

[nigelmegitt]

See also #711 and #769.

Currently two options are available for one component <position> values:

<position>
  : offset-position-h                             // single component value
  | offset-position-v                             // single component value

...

offset-position-h
  : position-keyword-h
  | <length>

offset-position-v
  : position-keyword-v
  | <length>

...

position-keyword-h
  : "center"
  | edge-keyword-h

position-keyword-v
  : "center"
  | edge-keyword-v

edge-keyword-h
  : "left"
  | "right"

edge-keyword-v
  : "top"
  | "bottom"

And the table explaining the equivalents of the one component values maps:

• center -> center center
• <length> -> <length> center

Both of those mappings indicates that a valid value for an offset-position-v of center or <length> will be interpreted as an offset-position-h. So in those cases the spec is actively misleading in suggesting that a vertical offset value can be used on its own.

The only one component values that are unambiguously vertical positions are top and bottom. Rather than handling those two specific values (which I guess would be rarely used because mostly authors leave a gap at the top or the bottom of the root container region), for simplicity I propose that we remove offset-position-v from the permitted set of one component values.

[skynavga]

Rather than dropping single value top and bottom, I suggest we replace offset-position-v with
edge-keyword-v in this context, which will make this an editorial change only.