WebGLBackground: prevent background material recompile in certain cases

[WestLangley]

Typically, a single renderer is used to render not only the scene, but for inset viewports, for render-targets, for post-processing, etc. In each case, the rendered scene may or may not have a background. Also, there may be multiple scenes. This can cause the scene background material to be recompiled every frame. (To replicate, add a texture background in /examples/webgl_shadowmap_viewer.html.)

I am thinking webGLBackground may have to be redesigned, but this change in an improvement -- at least in the cases I have studied.

[WestLangley]

Let's go with this for now. If we encounter additional edge cases, we can revisit this.

[mrdoob]

Thanks!