Tiny adjustment to SetScrollMargin

[scottbilas]

I noticed that the minimum size supported by a scroll region is 2 lines. Docs for DECSTBM say top must be less than bottom, not less-or-equal.

So this little check could use adjusting:

https://github.com/vezel-dev/cathode/blob/6bd76d8836c1988d96c58283de037de63f5e8ab9/src/core/Text/Control/ControlBuilder.cs#L404