frontend: release widget OpenGL resources before the context is destroyed

igexplorer segfaulted on close: the static Context outlived the stack-local
Application, so RenderWidget was destroyed during exit handlers and its
GLTexture members called glDeleteTextures through invalidated glad pointers.

Drop the Context references while the Application is alive, and clear the
Application children before ui::shutdown. Both owners must release early;
either one alone keeps the widgets alive past glfwTerminate.

Regression from the SDL2 to GLFW migration.

Author: PearCoding

src/frontend/explorer/main.cpp

@@ -58,6 +58,21 @@ class Context {
         app->setDropCallback(openFileCallback);
     }
 
+    // Must be called while the Application, and therefore the OpenGL context, is still alive
+    void teardown()
+    {
+        mRegistryWidget.reset();
+        mRegistryMenuItem.reset();
+        mOverviewWidget.reset();
+        mOverviewMenuItem.reset();
+        mParameterWidget.reset();
+        mParameterMenuItem.reset();
+        mHelpControlWidget.reset();
+        mHelpAboutWidget.reset();
+        mRenderWidget.reset();
+        mApplication = nullptr;
+    }
+
     static inline Context& instance()
     {
         static Context sContext;
@@ -190,6 +205,7 @@ int main(int argc, char** argv)
 
         const int exitcode = app.exec() ? EXIT_SUCCESS : EXIT_FAILURE;
         Context::instance().renderWidget()->cleanup();
+        Context::instance().teardown();
 
         return exitcode;
     } catch (const std::exception& e) {

src/frontend/ui/Application.cpp

@@ -42,6 +42,8 @@ class ApplicationInternal {
 
     ~ApplicationInternal()
     {
+        // Widgets may own OpenGL resources, which must be released while the context is still current
+        mChildren.clear();
         ui::shutdown(mWindow);
     }