Border top-right and bottom-right doesn't render properly when using a wide rune

[knz]

When I use a wide rune (e.g. "⏩") as a top-right or bottom-right corner, the resulting edge is wider than the other lines.

This should be corrected by padding the other lines.

See this example:

Found while working on #109.

cc @meowgorithm

[knz]

Interestingly, the issue does not happen if it's the bottom-left or top-left rune that's wide. For example:

set
border: border("","","","","⏩","","","")
----
⏩········␤
·hello····␤
·universe·␤
·!!·······␤
··········␤
··········🛇

set
border: border("","","","","","","","⏩")
----
··········␤
·hello····␤
·universe·␤
·!!·······␤
··········␤
⏩········🛇