Fixed Tiberian Sun construction yard flickering

as described in #5765.
Both
`Offset = new float2((float)(x + ((width / 2) - (frameSize.Width / 2))), (float)(y + ((height / 2) - (frameSize.Height / 2))));`
and
`Offset = new float2((float)(x + ((width / 2) - (0.5f * frameSize.Width))), (float)(y + ((height / 2) - (0.5f * frameSize.Height))));`
seem to work well.