Compositing shaders improvements, “Mountains of Fire” demo updated

[DP News-Robot]

I have improved the available “plugs” for our compositing shaders mechanism and updated their documentation. The overall goal was to make them friendly when you target only Phong shading, or want to switch between Phong and Gouraud shading. New PLUG_fragment_modify, simplest way to modify calculation after lighting, textures, fog are applied. This is also used …