python: properly provide virtual destructors for Application classes.

This time I actually have to thank you, Clang.

src/python/magnum/platform/egl.cpp

@@ -58,6 +58,10 @@ void egl(py::module& m) {
             #pragma GCC diagnostic pop
             #endif
         }
+
+        /* The base doesn't have a virtual destructor because in C++ it's never
+           deleted through a pointer to the base. Here we need it, though. */
+        virtual ~PyWindowlessApplication() {}
     };
 
     py::class_<PyWindowlessApplication> windowlessEglApplication{m, "WindowlessApplication", "Windowless EGL application"};

src/python/magnum/platform/glfw.cpp

@@ -45,6 +45,10 @@ void glfw(py::module& m) {
         void mousePressEvent(MouseEvent&) override {}
         void mouseReleaseEvent(MouseEvent&) override {}
         void mouseMoveEvent(MouseMoveEvent&) override {}
+
+        /* The base doesn't have a virtual destructor because in C++ it's never
+           deleted through a pointer to the base. Here we need it, though. */
+        virtual ~PublicizedApplication() {}
     };
 
     struct PyApplication: PublicizedApplication {

src/python/magnum/platform/glx.cpp

@@ -58,6 +58,10 @@ void glx(py::module& m) {
             #pragma GCC diagnostic pop
             #endif
         }
+
+        /* The base doesn't have a virtual destructor because in C++ it's never
+           deleted through a pointer to the base. Here we need it, though. */
+        virtual ~PyWindowlessApplication() {}
     };
 
     py::class_<PyWindowlessApplication> windowlessGlxApplication{m, "WindowlessApplication", "Windowless GLX application"};

src/python/magnum/platform/sdl2.cpp

@@ -45,6 +45,10 @@ void sdl2(py::module& m) {
         void mousePressEvent(MouseEvent&) override {}
         void mouseReleaseEvent(MouseEvent&) override {}
         void mouseMoveEvent(MouseMoveEvent&) override {}
+
+        /* The base doesn't have a virtual destructor because in C++ it's never
+           deleted through a pointer to the base. Here we need it, though. */
+        virtual ~PublicizedApplication() {}
     };
 
     struct PyApplication: PublicizedApplication {