Single-Pass OmniDirectional Shadows in a Deferred Shading Environment

Until recently, I believed that the only way to achieve omidirectional shadows was to use six passes to generate the faces of a cubemap, or to use two passes (one with MRT), to generate the faces of a dual paraboloid.

It turns out that we can in fact, attach a cubemap to a framebuffer such that we may draw to all six faces in a single pass, without using MRT, and the trick is to use a Geometry Shader (inbetween the VS and FS shader stages) whose job is to transform the shadowcaster geometry we're drawing , via one of six virtual camera views, into one of six output FS executions(!)
Our utilization of a Geometry Shader for 'multiple projections' of the rendered geometry leads to the setting of a built-in GLSL variable called gl_Layer, which is a zero based integer whose value can be read from the FS, indicating which CubeMap Face (or plane) is the target of the FS execution.

Unfortunately, unlike the Lighting stage, we can't use GPU Instancing to help with batching our lights - we can use it for drawing geometry to the shadow cubemap, but not to send the lights themselves.

So we have a choice then, to either perform a shadowing pass per light, or to write several shadowmap shader variants that each cope with a specific number of lights, just like we'd be doing if this was a Forwards rendering environment.

Due to the cost of shadowcasting, and in order to reduce the amount of work involved therein, I've introduced a boolean switch in the Material class, which denotes whether objects made of a given material can cast shadows. This will allow the user to make premium decisions about which elements of a scene may cast shadows, and reduce the average number of potential shadowcasters per frame.
If required, I can put an override in the RenderableComponent which will allow the user to allow or disallow shadowcasting on a per-renderable level and without regard to material settings.