Fix video mode 0 not getting centered

Fixes the issue where the first mode on the video mode list (1920x1200) does not get centered.

The changes in commit abe35fd0 in !1264 (merged) mean that VID_CheckRenderer in VID_SetMode uses the value of setmodeneeded, but this gets decremented in screen.c:220, resulting in incorrect behaviour when switching to video mode 0.