Border size computations look wrong

[knz]

1. GetBorderTopSize and GetBorderBottomSize returns the maximum rune width of all the border characters. This is wrong: the top/bottom border sizes are just 1 (1 line)

2. GetBorderLeftSize and GetBorderRightSize are using maxRuneWidth() on every part, but they should use runewidth.StringWidth() instead.

Found while working on #109