Make "Prefer smoother animations" default latency policy
At the moment, the Aurorae decoration engine is inefficient. With the medium latency policy, kwin misses a lot of vblanks when the decoration texture atlas needs to be updated. To work around it, this patch changes the default latency policy to "prefer smoother animations" to ensure that there is as much as possible time to repaint the screen.
Once Aurorae is optimized, we can change the default latency policy back to "medium."