How do I create a background material in Blender 2.8? Ultimately I want to export a glTF model that uses KHR_materials_unlit. This page says I should do that by using a background material.

Google tells me a lot about how to set the background image, but nothing I can find about how to create a "background material".  How do I do that?  I'm using Blender 2.8 because that's where the glTF exporter was introduced.
I discovered the answer on my own. Turns out that the "material" properties panel on the right side of the screen in shader view has a "Surface" property. Setting that to "Background" (and then rebinding the texture in the shader graph) will cause the GLTF exporter to use the KHR_materials_unlit extension.