Permit disabling of rounded corners on the edges between descendants

[ahfriedman]

Description:

Currently (swayfx version 0.5 (based on sway 1.10.1), if one uses a corner-radius and splits a stacked/tabbed window in multiple ways, the result will be as shown below where rounding is applied on each child independently—leaving gaps between them. I think it would be neat if there is a way to apply the rounding only to the top-level container as a whole—similar to the behavior of smart_corners, but on the container level instead of workspace so that way each stacked/tabbed container felt like a more unified collection of windows.