linux: Treat fullscreen as tiled on X11 and prevent resizing while maximized (#13990)

Two quick fixes for issues I noticed:

1. Fullscreening an unmaximized X11 window still showed rounded window
corners and allowed resizing
2. Maximized windows still allowed for resizing on corners due to
missing checks

![image](https://github.com/zed-industries/zed/assets/71973804/47df4de2-4013-4e51-88c3-d33b52a909f5)


Release Notes:

- N/A