Light Function and Post Process domain material thumbnails show the default material

Since 5.8, materials whose Material Domain is Light Function or Post Process no longer
preview correctly in the editor. They render as the default material (WorldGridMaterial)
instead of the material itself:

• Light Function: both the Content Browser thumbnail and the Material Editor preview
  viewport are affected.
• Post Process: only the Content Browser thumbnail is affected. The Material Editor
  preview is correct, because a Post Process material is previewed through an
  APostProcessVolume blendable rather than on the preview mesh.

This is a regression - the same materials preview correctly in 5.7. Light Function
rendering at runtime is unaffected, so the impact is limited to editor workflow: artists
can no longer identify or browse these materials by their auto-generated thumbnails.

Workaround
Adding the two domains to that check appears to restore the previous preview and
thumbnail behaviour:
Engine/Source/Runtime/Engine/Private/Materials/MaterialInterface.cpp (around line 3057)

// if (MaterialDomain != MD_Surface && MaterialDomain != MD_DeferredDecal && MaterialDomain != MD_Volume && MaterialDomain != MD_UI)
if (MaterialDomain != MD_Surface && MaterialDomain != MD_DeferredDecal && MaterialDomain != MD_Volume && MaterialDomain != MD_UI && MaterialDomain != MD_PostProcess && MaterialDomain != MD_LightFunction)

1. Create a new blank project
2. Create a Material, set Material Domain = Light Function.
3. Connect a bright Constant3Vector to Emissive Color, then Apply and Save.
4. Look at the Material Editor preview viewport and the Content Browser thumbnail.
5. Repeat with Material Domain = Post Process.

Expected: both show the material's emissive output, as in 5.7.
Observed: both render the default material (WorldGridMaterial checker).