A2C won't work in RTTs on Mesa
An OSG issue. Tracking upstream as https://github.com/openscenegraph/OpenSceneGraph/issues/1028.
Especially annoyingly, A2C falls back to being totally opaque when multisampling is off, so this is going to cause complaints. I've got a hack-fix locally, which I'll probably enable for older OSG versions, but would rather wait until we've got a version number for it being fixed upstream as it's really hacky and I don't want it in our code unfenced.