backends: Don't center the pointer on initialization

Centering the pointer at startup causes undesired behaviour if it ends up hovering over reactive elements, that might react to that positioning, causing confusion. This is the case of the login dialog when a list of different users is shown, as centering the pointer at startup in that case will get the user in the center of the screen pre-selected, which is not the expected behaviour (i.e. pre-selecting the first one).

Fix this by simply moving the pointer out of the way to the top-left corner during initialization.

Closes: gnome-shell#157 (closed)