Platform: explicitly destroy GL::Context before the GL context.

So in case it touches the GL state in some way, it doesn't do that on an
already destroyed context. The windowless apps do this all implicitly
due to the WindowlessGLContext encapsulation.

src/Magnum/Platform/AbstractXApplication.cpp

@@ -109,6 +109,8 @@ bool AbstractXApplication::tryCreate(const Configuration& configuration, const G
 }
 
 AbstractXApplication::~AbstractXApplication() {
+    /* Destroy Magnum context first to avoid it potentially accessing the
+       now-destroyed GL context after */
     _context.reset();
 
     /* Shut down context handler */

src/Magnum/Platform/AndroidApplication.cpp

@@ -77,6 +77,10 @@ AndroidApplication::AndroidApplication(const Arguments& arguments, NoCreateT): _
 }
 
 AndroidApplication::~AndroidApplication() {
+    /* Destroy Magnum context first to avoid it potentially accessing the
+       now-destroyed GL context after */
+    _context.reset();
+
     eglMakeCurrent(_display, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT);
     eglDestroyContext(_display, _glContext);
     eglDestroySurface(_display, _surface);

src/Magnum/Platform/EmscriptenApplication.cpp

@@ -260,6 +260,8 @@ EmscriptenApplication::EmscriptenApplication(const Arguments& arguments, NoCreat
 
 EmscriptenApplication::~EmscriptenApplication() {
     #ifdef MAGNUM_TARGET_GL
+    /* Destroy Magnum context first to avoid it potentially accessing the
+       now-destroyed GL context after */
     _context.reset();
 
     emscripten_webgl_destroy_context(_glContext);

src/Magnum/Platform/GlfwApplication.cpp

@@ -668,6 +668,12 @@ void GlfwApplication::setupCallbacks() {
 }
 
 GlfwApplication::~GlfwApplication() {
+    #ifdef MAGNUM_TARGET_GL
+    /* Destroy Magnum context first to avoid it potentially accessing the
+       now-destroyed GL context after */
+    _context.reset();
+    #endif
+
     glfwDestroyWindow(_window);
     for(auto& cursor: _cursors)
         glfwDestroyCursor(cursor);

src/Magnum/Platform/Sdl2Application.cpp

@@ -796,6 +796,8 @@ Sdl2Application::~Sdl2Application() {
        all. */
 
     #ifdef MAGNUM_TARGET_GL
+    /* Destroy Magnum context first to avoid it potentially accessing the
+       now-destroyed GL context after */
     _context.reset();
 
     #ifndef CORRADE_TARGET_EMSCRIPTEN