Animation: add TrackViewStorage::interpolator().

It was the last remaining field that wasn't queryable from this
type-erased base.

doc/changelog.dox

@@ -438,6 +438,9 @@ See also:
     @ref Animation::BasicEasing "easing functions" implemented with
     double precision, in addition to single-precision @ref Animation::Easing,
     and similar in spirit to the @ref Constants and @ref Constantsd typedefs.
+-   Added @ref Animation::TrackViewStorage::interpolator() for getting a
+    type-erased interpolator pointer without having to cast to a concrete
+    @ref Animation::TrackView type
 
 @subsubsection changelog-latest-changes-debugtools DebugTools library
 

src/Magnum/Animation/Test/TrackViewTest.cpp

@@ -569,6 +569,7 @@ void TrackViewTest::constructCopyStorage() {
         Extrapolation::Extrapolated, Extrapolation::DefaultConstructed};
 
     const TrackViewStorage<Float> b = a;
+    CORRADE_COMPARE(b.interpolator(), reinterpret_cast<void(*)()>(customLerp));
     CORRADE_COMPARE(b.keys().size(), 2);
     CORRADE_COMPARE(b.values().size(), 2);
     CORRADE_COMPARE(Containers::arrayCast<Float>(b.keys())[1], 5.0f);

src/Magnum/Animation/Track.h

@@ -482,6 +482,18 @@ template<class K> class TrackViewStorage {
             return _keys.size();
         }
 
+        /**
+         * @brief Type-erased interpolation function
+         * @m_since_latest
+         *
+         * Has to be cast to a type corresponding to the value type before use.
+         * Use @ref TrackView and @ref TrackView::interpolator() to get the
+         * interpolator function in a concrete type.
+         */
+        void(*interpolator() const)() {
+            return _interpolator;
+        }
+
         /**
          * @brief Key data
          *