changelog: document new igl functions per review

Author: jf---

CHANGELOG.md

@@ -9,6 +9,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+Add missing igl functions to enable compas_slicer to drop the `libigl` dependency entirely:
+
+- `cotmatrix` module: `trimesh_cotmatrix`, `trimesh_cotmatrix_entries`
+- grad module: `trimesh_grad` (gradient operator)
+- meshing module: `trimesh_cut_mesh`, `trimesh_face_components`
+- simplify module: `ramer_douglas_peucker` (polyline simplification)
+
 ### Changed
 
 ### Removed