Decoration: remove extra 1px padding
This fixes the 1px leak when a window is maximized on X11 with 200% scaling.
According to git history, the extra 1px was introduced from the beginning of Breeze to support "separator under active window's titlebar", and it's removed in Plasma 5.24.
Amends 660b7ecc
| Before |  |
| After |  |
Edited by Fushan Wen