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

This reverts commit dc2617130b, 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.

source/blender/blenloader/intern/writefile.c

@@ -679,8 +679,10 @@ static void write_iddata(void *wd, const ID *id)
 
 static void write_previews(WriteData *wd, const PreviewImage *prv_orig)
 {
-	/* Never write previews when doing memsave (i.e. undo/redo)! */
-	if (prv_orig && !wd->current) {
+	/* Note we write previews also for undo steps. It takes up some memory,
+	 * but not doing so would causes all previews to be re-rendered after
+	 * undo which is too expensive. */
+	if (prv_orig) {
 		PreviewImage prv = *prv_orig;
 
 		/* don't write out large previews if not requested */