Code review comment for lp:~kdub/mir/post-if-optimizable

Ah. Let me see if I've got how the nested case will work:

The inner-Mir will always accept whatever renderable list you care to pass to post_renderables_if_optimizable() and send them down to the outer-Mir. The outer-Mir will then post_renderables_if_optimizable() and either go “yay!” or do a GL composition pass.

And we optimise further the implementation of post_renderables_if_optimizable() running on the hardware will only bail if none of the renderables are optimisable.

So this makes it easier to bring up platforms in the short term, and will probably be a no-op in the long term?