Code review comment for lp:~kdub/mir/display-groups

fixed nits/tests I forgot to reenable.

As for the performance concern, thats true. Ideally two threads would draw, and then one thread would post. This is forced on us somewhat by the android api, although its mitigated by:
* Android (surfaceflinger) would have the same multimonitor performance hit, as it drives rendering from one thread as well.
* mesa, and the other platforms are still driven in sensible way we have it.
* iirc, most android gpus have 2 or more banks of pipeline/contexts for this scenario
* The bottleneck is probably the system bus, instead of the cpu assembly of the gl commands (which is what the two threads would help with)