Disable curved arrows when they go up

[dwhatley19]

https://web.mit.edu/dwhatley/www/snapdown/web/production.html#a%20-%3E%20(Main%20a%20-%3E%202%2C%20b%20-%3E%20c)%0Ac%20-%3E%20(Main2%20d%20-%3E%20a)

When an arrow starts below where it ends, we should probably preserve the polyline nature of the arrow, rather than trying to make it curve.

Might be useful for arrows from objects to stack frames.