Merged changes in the trunk up to revision 52191.

Conflicts resolved:
source/blender/makesdna/DNA_scene_types.h

source/blender/render/intern/source/pipeline.c

@@ -435,7 +435,10 @@ void RE_FreePersistentData(void)
 	/* render engines can be kept around for quick re-render, this clears all */
 	for (re = RenderGlobal.renderlist.first; re; re = re->next) {
 		if (re->engine) {
-			RE_engine_free(re->engine);
+			/* if engine is currently rendering, just tag it to be freed when render is finished */
+			if (!(re->engine->flag & RE_ENGINE_RENDERING))
+				RE_engine_free(re->engine);
+
 			re->engine = NULL;
 		}
 	}