Add hide_background option for envmaps

[georgy95]

During model rendering, I'd like to use the environment map for lighting purposes only (as an injection).

My goal is to render a 3D model in 360 view, without any background, which is automatically set by the envmap.

As is stands, we need to add additional 3D objects into the scene to fill the background space.

[njroussel]

Hi @skiler07

Most integrators have a parameter hide_emitters (example in direct). When enabling this, it should behave as you describe -- of course this assumes the only directly visible source of light in your scene is the environment map.

[Speierers]

Although I think this option is still missing in some integrators (e.g. prb).