Add antialiasing to rounded corners.

[enderprism]

I'm submitting a…

[ ] Bug
[x] Feature Request
[ ] Documentation Request
[ ] Other (Please describe in detail)

Current Behavior

Rounded corners look pixelated, and not antialiased.

Desired Behavior

Add an option to enable antialiasing on rounded corners.

Impact

[X] This feature requires new configuration and/or commands

Environment

Output of i3 --moreversion 2>&-:

i3 version: 
Binary i3 version:  4.20.1-174-g95333775 © 2009 Michael Stapelberg and contributors
Running i3 version: 4.20.1-174-g95333775 (pid 345164)
Loaded i3 config:
  /home/username/.config/i3/config (main) (last modified: jeu. 20 avril 2023 17:39:34, 1127 seconds ago)

The i3 binary you just called: /usr/bin/i3
The i3 binary you are running: i3

- Linux Distribution & Version: EndeavourOS (Arch Linux)
- Are you using a compositor (e.g., xcompmgr or compton): Picom (Pijulius fork)