SV_HullForBsp: use hull center instead of corner

[WPMGPRoSToTeMa]

If bbox size greater than hull size:
with old code: hull is located in bbox negative corner (-x and -y).
with new code: hull is located in bbox center.

[IgnacioFDM]

What difference does this make in-game?

[theAsmodai]

This will affect to movement of entities like missiles.
@WPMGPRoSToTeMa, yes, I think a cvar is desirable.

[WPMGPRoSToTeMa]

64x64 (from -32 to 32):

128x128 (from -64 to 64) sv_rehlds_hull_centering 0:

128x128 (from -64 to 64) sv_rehlds_hull_centering 1:

[hajimura]

@WPMGPRoSToTeMa, nicely done