WIP: Transformation of instances in world coordinate space:

Ulocking the "anisotropic" scaling checkbox will bake the transformation
into meshes to allow for scaling in world axes.

Optimized and templated the stl_transform functions, now also available for 3x3 matrices.
The Canvas3D::reload_scene() now maintains selection even if all volumes of an instance changed their IDs.

src/libslic3r/TriangleMesh.hpp

@@ -52,6 +52,7 @@ public:
     void mirror_y() { this->mirror(Y); }
     void mirror_z() { this->mirror(Z); }
     void transform(const Transform3d& t, bool fix_left_handed = false);
+	void transform(const Matrix3d& t, bool fix_left_handed = false);
     void align_to_origin();
     void rotate(double angle, Point* center);
     TriangleMeshPtrs split() const;