The same as center_blit(), but only centers horizontally.

def hcenter_blit(target, source, dest = (0, 0), area=None, special_flags=0):
    '''
    The same as center_blit(), but only centers horizontally.
    '''
    loc = lambda d, s: (_vec(d.get_width() / 2, 0) - 
            _vec(s.get_width() / 2, 0))
    _blitter(loc, target, source, dest, area, special_flags)