Corrected openGL framebuffer releasing.

modules/juce_opengl/opengl/juce_OpenGLContext.cpp

@@ -323,6 +323,7 @@ public:
         if (context.renderer != nullptr)
             context.renderer->openGLContextClosing();
 
+        cachedImageFrameBuffer.release();
         nativeContext->shutdownOnRenderThread();
 
         associatedObjectNames.clear();