Fix the middle/right button confusion in the scroll button assignment
In X, buttons 1, 2, 3 are left, middle, right. In evdev, the order is BTN_LEFT, BTN_RIGHT, BTN_MIDDLE. So setting a scroll button to 2 gave us a middle button in the X session and a right button in a wayland session.
Fix that by hard-coding the LMR buttons handling.
Edited by Peter Hutterer