Update Mega Texture

Here we have added automated occluders for GPU culling so that there is a reduction in draw calls for objects. Note that this only works for the DX11 library because when performing a downsampling operation DX11 can contain shader resource view so that you can sample from and render to different levels in the same Mip chain. DX9 you would have to copy a separate render target into the Mipmap because you couldn't isolate a single Mip level in the same chain.