Fix bglayer y position, now it uses the same logic as x, as well as entities.
gfx_y_offset no longer includes advancey, instead it works exactly the same way as gfx_x_offset.
The macro screenx and screeny in openbor.h now works both with and without level (as long as elevator level logic is not merged, they are necessary).
Authored by: utunnels 2011-10-27
Parent: [r3561]
Child: [r3563]
