fix for bug 490: ensure intro movie is centered on linux

revert atlimit8's commit from feef4e844a1d4cc0ba6d38049ada58ac539bf4a9
I attempted to reach him/her about the reasoning for this commit but did
not get a reply.

tested on Gentoo Linux with sdl-1.2.15, sdl-gfx-2.0.24 with:
  displayWidth:    1920
  displayHeight:   1200
  baseXResolution: 640
  baseYResolution: 400

see:
https://github.com/SupSuper/OpenXcom/commit/feef4e844a1d4cc0ba6d38049ada58ac539bf4a9
https://github.com/SupSuper/OpenXcom/pull/678
https://github.com/SupSuper/OpenXcom/pull/675
http://openxcom.org/bugs/openxcom/issues/294
http://openxcom.org/bugs/openxcom/issues/490