Fix T51412: Instant crash with texture plugged into the Displacement output

The issue was caused by unlimited textures commit, root of the issue is that
displacement code updates some of the image slots directly, so it needs to
ensure device vectors are all proper size.

intern/cycles/render/mesh.cpp

@@ -1944,6 +1944,7 @@ void MeshManager::device_update_displacement_images(Device *device,
 			}
 		}
 	}
+	image_manager->device_prepare_update(dscene);
 	foreach(int slot, bump_images) {
 		pool.push(function_bind(&ImageManager::device_update_slot,
 		                        image_manager,