Trade: convenience default constructor for AnimationTrackData.

Otherwise one would need to use a NoInit Array constructor and that
would cause dangling deleter function pointer call after the plugin gets
unloaded.

src/Magnum/Trade/AnimationData.h

@@ -163,6 +163,15 @@ API.
 */
 class AnimationTrackData {
     public:
+        /**
+         * @brief Default constructor
+         *
+         * Leaves contents at unspecified values. Provided as a convenience for
+         * initialization of @ref AnimationData structure, expected to be
+         * replaced with concrete values later.
+         */
+        /*implicit*/ AnimationTrackData() noexcept: _type{}, _resultType{}, _target{}, _targetId{}, _view{} {}
+
         /**
          * @brief Constructor
          * @param type          Value type

src/Magnum/Trade/Test/AnimationDataTest.cpp

@@ -38,6 +38,8 @@ struct AnimationDataTest: TestSuite::Tester {
     void constructCopy();
     void constructMove();
 
+    void constructTrackDataDefault();
+
     void trackCustomResultType();
 
     void trackWrongIndex();
@@ -53,6 +55,8 @@ AnimationDataTest::AnimationDataTest() {
               &AnimationDataTest::constructCopy,
               &AnimationDataTest::constructMove,
 
+              &AnimationDataTest::constructTrackDataDefault,
+
               &AnimationDataTest::trackCustomResultType,
 
               &AnimationDataTest::trackWrongIndex,
@@ -225,6 +229,11 @@ void AnimationDataTest::constructMove() {
     }
 }
 
+void AnimationDataTest::constructTrackDataDefault() {
+    AnimationTrackData data;
+    CORRADE_VERIFY(true); /* no public accessors here, so nothing to check */
+}
+
 void AnimationDataTest::trackCustomResultType() {
     using namespace Math::Literals;