wayland: add mouse buttons and fix axis scaling

Previously, the only mouse buttons supported in wayland were left,
right, and middle click. This adds the thumb back/forward buttons as
valid bindings. Also it removes the old, default behavior of always
sending a right click if an unrecognized mouse button is clicked.
In a related but different fix, the magnitude of an axis event in
wayland is not important to mpv since it internally handles all scaling.
The only thing we care about is getting the sign when the event occurs.