Fixed unintialized variable.

Accidentaly found when running unit tests on GLES build.

src/Physics/ShapedObjectGroup.h

@@ -40,6 +40,13 @@ template<std::uint8_t dimensions> class PHYSICS_EXPORT ShapedObjectGroup {
     friend class ShapedObject<dimensions>;
 
     public:
+        /**
+         * @brief Constructor
+         *
+         * Marks the group as dirty.
+         */
+        inline constexpr ShapedObjectGroup(): dirty(true) {}
+
         /**
          * @brief Destructor
          *