display-settings: Switch to one-dimensional scaling
Only one dimension was actually used in the UI, as the two spin buttons were synchronized. Furthermore, Wayland only supports one-dimensional scaling.
The x/y data structures of the X11 implementation are retained, however, since randr supports this, and because this could be leveraged later to match Wayland's notion of custom mode.
Related: !74 (comment 50071)
Related: #523
Edited by Gaël Bonithon