deco: Implement "should corner be rounded" detection

[Javyre]

Currently, corner radii >0 are pretty ugly and useless since it is blindly applied to every corner. We need to detect whether a corner should be rounded or not so as to avoid "holes" as described in #19 (comment)

[ErikReider]

So this should only round the outside corners (the corners closest to each display edge)?

[Javyre]

A setting could be added to round display-edge corners pretty easily, but in f1157e9 it only rounds outer corners of floating nodes (either single floating views or whole floating tiling trees)

[ErikReider]

Oh. I read my comment again and it didn't really make any sense 😅. I was wondering if this PR only rounds the corners of tiled windows that are the closest to each corner of the display to remove the gap in between each window

[Javyre]

Oh I see. Yes exactly. Except that the main tiling tree (i.e. non-floating trees) aren't rounded at all. (this last part is what I was saying could be made configurable if there is demand)