message: macOS: add cleanup for observer.
diff: @@ -36,6 +36,7 @@ using namespace backend;
  struct PlatformCocoaGLImpl {
  NSOpenGLContext* mGLContext = nullptr;
  NSView* mCurrentView = nullptr;
  + NSObject* mObserver = nullptr;
  std::vector<NSView*> mHeadlessSwapChains;
  };
  @@ -71,7 +72,7 @@ Driver* PlatformCocoaGL::createDriver(void* sharedContext) noexcept {
  int result = bluegl::bind();
  ASSERT_POSTCONDITION(!result, "Unable to load OpenGL entry points.");
  - [
  + pImpl->mObserver = [
  [NSNotificationCenter defaultCenter] addObserverForName: NSViewGlobalFrameDidChangeNotification
  object: nil
  queue: nil
  @@ -84,6 +85,7 @@ Driver* PlatformCocoaGL::createDriver(void* sharedContext) noexcept {
  }
  void PlatformCocoaGL::terminate() noexcept {
  + [[NSNotificationCenter defaultCenter] removeObserver: pImpl->mObserver];
  pImpl->mGLContext = nil;
  bluegl::unbind();
  }
  