Fix the middle/right button confusion in the scroll button assignment

Merged Peter Hutterer requested to merge whot/mutter:wip/scroll-button-right-confusion into master

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

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