wayland: Fix incorrect button mask in clutter_wayland_handle_button

Copied from https://bugzilla.gnome.org/show_bug.cgi?id=783791#c3 with rebased patches.

While browsing through the code to diagnose an unrelated issue, I noticed that Clutter is using the wrong mask when a middle- or right-click is received. See link. Evdev event codes 272 (BTN_LEFT), 273 (BTN_RIGHT), and 274 (BTN_MIDDLE) are assigned the proper clutter button number, but the mask is incorrect.

I'm unsure what real-world effects this has, but I have confirmed with Carlos Garnacho in a private email that the code is indeed incorrect.