impl Default for gltf_json::Node.

Since `Node`s are generally expected to have only a few non-absent
fields, providing this `Default` implementation will allow code that is
generating `Node`s to be much more concise.

gltf-json/src/scene.rs

@@ -13,7 +13,7 @@ use serde_derive::{Deserialize, Serialize};
 /// identity. When a node is targeted for animation (referenced by an
 /// animation.channel.target), only TRS properties may be present; `matrix` will not
 /// be present.
-#[derive(Clone, Debug, Deserialize, Serialize, Validate)]
+#[derive(Clone, Debug, Default, Deserialize, Serialize, Validate)]
 pub struct Node {
     /// The index of the camera referenced by this node.
     #[serde(skip_serializing_if = "Option::is_none")]