Fix and clean up skeletal animation (#15722)

* Fix attachments lagging behind their parents (#14818)
* Fix animation blending (#14817)
* Bring back cool guy as another .x smoke test
* Add .x mesh loader unittest
* Do bounding box & matrix calculation at proper point in time
* Remove obsolete `SAnimatedMesh`

irr/include/IAnimatedMesh.h

@@ -13,8 +13,8 @@ namespace scene
 //! Interface for an animated mesh.
 /** There are already simple implementations of this interface available so
 you don't have to implement this interface on your own if you need to:
-You might want to use irr::scene::SAnimatedMesh, irr::scene::SMesh,
-irr::scene::SMeshBuffer etc. */
+You might want to use irr::scene::SMesh, irr::scene::SMeshBuffer etc.
+*/
 class IAnimatedMesh : public IMesh
 {
 public:
@@ -34,22 +34,8 @@ public:
 	scene node the mesh is instantiated in.*/
 	virtual void setAnimationSpeed(f32 fps) = 0;
 
-	//! Returns the IMesh interface for a frame.
-	/** \param frame: Frame number, >= 0, <= getMaxFrameNumber()
-	Linear interpolation is used if this is between two frames.
-	\return Returns the animated mesh for the given frame */
-	virtual IMesh *getMesh(f32 frame) = 0;
-
-	//! Returns the type of the animated mesh.
-	/** In most cases it is not necessary to use this method.
-	This is useful for making a safe downcast. For example,
-	if getMeshType() returns EAMT_MD2 it's safe to cast the
-	IAnimatedMesh to IAnimatedMeshMD2.
-	\returns Type of the mesh. */
-	E_ANIMATED_MESH_TYPE getMeshType() const override
-	{
-		return EAMT_UNKNOWN;
-	}
+	//! Returns the type of the animated mesh. Useful for safe downcasts.
+	E_ANIMATED_MESH_TYPE getMeshType() const = 0;
 };
 
 } // end namespace scene