one-off rendering error in rounded backdrops

[disconcision]

by the cursor.

proposal: in the layout code, i think that right now each row has two {-1,0,1}s indicating whether the above/below rows are shorter/equal/longer. replace this with the actual difference. the code that conditions on it right now should condition on the sign. in the case that the difference is less than the curvature radius, do two things:

1. make the bezier endpoint half way towards above/below neighbor (currently this is radius)
2. make the bezier angle at that point 45 degrees (currently this is horizontal) to meet up smoothly with neighbor.