Blender Git Loki

Fix part of T53080: all material previews re-rendering after undo.

This reverts commit dc2617130b2e1d7d2b9892fbd7c6e7b60caafb66, which disabled
writing of previews for undo. While this uses some memory, re-rendering all
previews is very expensive, especially if for example you have lots of materials
using high-res image textures.