since 2.5.2 lightgrid common shader doesnt work anymore.
here are results form a test map:
lightgrid brush is 1024x1024x512 with 2.5.3
31 x 31 x 7 = 6727 grid
lightgrid brush is 1024x1024x512 with 2.3.32
17 x 17 x 5 = 1445 grid
