Fix T55436: crash with point density and shader tweaking.

With copy-on-write we can no longer assumes the point density data is
available after Cycles synchronization with Blender data is done. So
force it to be loaded earlier, even if it's not great for interactivity.

intern/cycles/render/image.cpp

@@ -943,6 +943,39 @@ void ImageManager::device_update_slot(Device *device,
 	}
 }
 
+void ImageManager::device_load_builtin(Device *device,
+                                       Scene *scene,
+                                       Progress& progress)
+{
+	/* Load only builtin images, Blender needs this to load evaluated
+	 * scene data from depsgraph before it is freed. */
+	if(!need_update) {
+		return;
+	}
+
+	TaskPool pool;
+	for(int type = 0; type < IMAGE_DATA_NUM_TYPES; type++) {
+		for(size_t slot = 0; slot < images[type].size(); slot++) {
+			if(!images[type][slot])
+				continue;
+
+			if(images[type][slot]->need_load) {
+				if(images[type][slot]->builtin_data) {
+					pool.push(function_bind(&ImageManager::device_load_image,
+					                        this,
+					                        device,
+					                        scene,
+					                        (ImageDataType)type,
+					                        slot,
+					                        &progress));
+				}
+			}
+		}
+	}
+
+	pool.wait_work();
+}
+
 void ImageManager::device_free_builtin(Device *device)
 {
 	for(int type = 0; type < IMAGE_DATA_NUM_TYPES; type++) {