Resolve clippy::needless_pass_by_ref_mut warning

```
warning: this argument is a mutable reference, but not used mutably
  --> src/collada/image.rs:78:24
   |
78 | fn parse_image<'a>(cx: &mut Context<'a>, node: xml::Node<'a, '_>) -> io::Result<Image<'a>> {
   |                        ^^^^^^^^^^^^^^^^ help: consider changing to: `&Context<'a>`
   |
   = note: this is cfg-gated and may require further changes
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
   = note: `-W clippy::needless-pass-by-ref-mut` implied by `-W clippy::nursery`
   = help: to override `-W clippy::nursery` add `#[allow(clippy::needless_pass_by_ref_mut)]`
```

src/collada/image.rs

@@ -75,7 +75,7 @@ pub(super) fn parse_library_images<'a>(
     Ok(())
 }
 
-fn parse_image<'a>(cx: &mut Context<'a>, node: xml::Node<'a, '_>) -> io::Result<Image<'a>> {
+fn parse_image<'a>(cx: &Context<'a>, node: xml::Node<'a, '_>) -> io::Result<Image<'a>> {
     debug_assert_eq!(node.tag_name().name(), "image");
     let id = node.required_attribute("id")?;
     // let name = node.attribute("name");