Renderer should not be accessible as a field on Compositor

[Timidger]

With the addition of DMA buffers (and probably output damage later) there needs to be pointers from the renderer living elsewhere. Since the renderer is optional, it is publically acessible as an Option<GenericRenderer> so at any time the user could destroy it by storing a None in there. Not good if we don't want dangling pointers.

To get around this we will need to add getters so that it's impossible to set the renderer after creation.