RTT mip-maps are not being generated automatically on D3D12

[docEdub]

Running playground https://playground.babylonjs.com/#W7E7CF#25 on D3D12 shows the following when the window is small or the camera is moved far enough away...

It should look like this ...

Cedric and I looked at this in RenderDoc and saw the RTT mip-maps are missing on D3D12. They are generated automatically for other graphics APIs, but not D3D12.