Fix crash after OpenGL::detach in the presence of buffered child components

The crash fixed by this commit could be triggered by attaching an OpenGL
context to a component, calling setBufferedToImage (true) on one of its
child components, and then detaching the OpenGL context from the parent.

Since 4ba01a8 we are creating images
with the current rendering context's native image type, so the above
scenario would leave an image buffer that references the detached
context.

Author: szarvas

modules/juce_opengl/opengl/juce_OpenGLContext.cpp

@@ -1102,11 +1102,19 @@ class OpenGLContext::Attachment final : public ComponentMovementWatcher,
         detach();
     }
 
+    static void clearCachedImagesInComponentTree (Component& root)
+    {
+        root.setCachedComponentImage (nullptr);
+
+        for (auto* child : root.getChildren())
+            clearCachedImagesInComponentTree (*child);
+    }
+
     void detach()
     {
         auto& comp = *getComponent();
         stop();
-        comp.setCachedComponentImage (nullptr);
+        clearCachedImagesInComponentTree (comp);
         context.nativeContext = nullptr;
     }