Add mechanism for gathering lights and feeding into shaders

Once the lights have a spatial extent (see https://bugreports.qt.io/browse/QTBUG-48833) it should be possible to automatically gather the most important lights for a given object and pass the light info through to the pipeline for rendering.

This may require a new framegraph node that determines how the light information is collected. E.g. grab the brightest 3 lights at the object's world position.