Fix T46729: Render result different between animated and still renders

The issue was in fact a feedback loop between render database init which needs
to know which render layers to initialize and scene update for newframe which
needs to know which layers to update.

Worked around by updating scene animation before render database initialization
which isn't really ideal, but doesn't really have much better alternative.

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

@@ -35,6 +35,7 @@
 #include <stdlib.h>
 #include <stddef.h>
 
+#include "DNA_anim_types.h"
 #include "DNA_image_types.h"
 #include "DNA_node_types.h"
 #include "DNA_object_types.h"
@@ -3608,7 +3609,24 @@ void RE_BlenderAnim(Render *re, Main *bmain, Scene *scene, Object *camera_overri
 	else {
 		for (nfra = sfra, scene->r.cfra = sfra; scene->r.cfra <= efra; scene->r.cfra++) {
 			char name[FILE_MAX];
-			
+
+			/* Here is a feedback loop exists -- render initialization requires updated
+			 * render layers settings which could be animated, but scene evaluation for
+			 * the frame happens later because it depends on what layers are visible to
+			 * render engine.
+			 *
+			 * The idea here is to only evaluate animation data associated with the scene,
+			 * which will make sure render layer settings are up-to-date, initialize the
+			 * render database itself and then perform full scene update with only needed
+			 * layers.
+			 *                                                              -sergey-
+			 */
+			{
+				float ctime = BKE_scene_frame_get(scene);
+				AnimData *adt = BKE_animdata_from_id(&scene->id);
+				BKE_animsys_evaluate_animdata(scene, &scene->id, adt, ctime, ADT_RECALC_ALL);
+			}
+
 			/* only border now, todo: camera lens. (ton) */
 			render_initialize_from_main(re, &rd, bmain, scene, NULL, camera_override, lay_override, 1, 0);