Scene graph

[Daniel van Vugt]

OK, I admit it. Recalculating the scene graph elements to display on 
every frame is now hurting. If you add up the bottlenecks found so far 
it accounts for around 20% of Mir's CPU usage (with a low number of 
surfaces even).

https://bugs.launchpad.net/mir/+bugs?field.tag=performance

Seems like we need to move to the model of "everything in a 
precalculated scene graph" ASAP. One where the scene graph changes much 
less often than the frame rate. Although that might be over-ambitious to 
complete in the next month or two.

- Daniel