AudioProcessorEditor drawing with OpenGLRenderer flickers on resizing

OpenGL is often tricky to embed without artefacts when the window is resized. In an app it's possible to use some tricks to work around this, by forcing synchronous updates when the window changes, but in plugins where the window is owned by the host, it's harder. I can't justify us spending (possibly quite a long) time investigating this, when it's far from certain that anything could be done to prevent it.