does not respect window size (cannon moves outside mountains)

[jwaldmann]

When I resize (increase width) the window in which the game runs, then the mountain range stays the same size (so, part of the window is black) but the "cannon" (the thing that fires the bullets) will move over the full width (so, outside mountains).

This is particularily bad under XMonad (tiling window manager) because the size of the window will be determined according to (current) tiling, and not to preferences of the application.

(SpaceInvaders-0.4.5 (current master from git), X11-1.8, Yampa-0.10.6.2, HGL-3.2.3.1, ghc-8.2.1, fedora 26)