New modifier for volumetrics: Density

As far as shader nodes can't be converted to 3D texture or can't rule volumetric before I convert it to mesh with modifier, I propose new Density modifier for volumetric objects.

It will provide easier way to create meshes with weird complex structures.

Now displace XYZ modifier only work with positive color values, that prevents to have negative displacement.

Of course it is more desirable to have something like volumetric baking or shader-to-volume-to-mesh direct pipeline.