Area Lights

## What problem does this solve or what need does it fill?

Adds the remaining common type of light, alongside the existing directional/point/emissive/spotlights.

## Research

* [Moving Frostbite to Physically Based Rendering 3.0](https://seblagarde.wordpress.com/2015/07/14/siggraph-2014-moving-frostbite-to-physically-based-rendering/)
* [Real Shading in Unreal Engine 4](https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf)
* [A Journey Through Implementing Multiscattering BRDFs and Area Lights](https://advances.realtimerendering.com/s2019/A%20Journey%20Through%20Implementing%20Multiscattering%20BRDFs%20and%20Area%20Lights.pptx) (note that there are speaker notes embed in the slides)

Start by reviewing the existing directional, point, emissive, and spotlight implementations, then read the linked physically based rendering and area-light research. Done means the engine supports area lights alongside those existing light types, but this issue does not identify files, tests, or a more specific acceptance criterion.

Written by the indexing model from the issue text.