Add a virtual destructor for ColourProvider

Author: tmaier-sc

components/pango_opengl/include/pangolin/gl/colour.h

@@ -148,6 +148,9 @@ class ColourProvider
 
     /// Reset colours
     virtual void Reset() = 0;
+
+    /// Virtual destructor to ensure correct destruction of derived classes
+    virtual ~ColourProvider() = default;
 };
 
 /// A ColourWheel is like a continuous colour palate that can be sampled.