GLSL shaders

[so19]

In the latest test build I can see there is shader support and new shaders can be defined in shaders.ddf but how does the engine know when to use a custom shader?

For example if I wanted a custom shader to be deployed all the time how would this work? Equally say I had a GLSL shader to render a more realistic NVG effect how would I “link” this to be used when the night vision goggles special item is picked up?