Stencil Buffer: Advanced Managing by WebGLRenderer

[Mugen87]

After providing a basic stencil buffer API in WebGLState(see #7816), i tried to implement an automatic management of the stencil test (see #7836). The WebGLRenderer should change the WebGL state by two additional material properties stencilWrite and stencilTest. The benefit of this is a more consistent management of the internal WebGL state.

Unfortunately, the masking mechanism of three.js's post processing was incompatible with this approach, because the library, or more specifically the MaskPass, manages directly the stencil test via native WebGL API. I tried to fix this (see #7866), but i wasn't able to develop a satisfying solution.

I've opened primarily this issue as a reminder. But we can also discuss here all suggestions to this topic.