Consider adding attachable image buffers to CustomMaterial

In current implementation postprocessing is limited and things like Mesh outlining can't be implemented.

I suggest that attachable image buffers should be implemented for CustomMaterial.

It will be possible to write additional information from CustomMaterial's fragment shaders to these buffers. In such buffers can be written, for instance, if particular fragment belongs to rendered mesh i.e. it can be used as stencil buffer. Then in postprocessing it would be possible to outline all fragments which are not black in that buffer.