OSX: Lower framerate from 25FPS to 10FPS

This is the only thing we can do to lower the CPU consumption of the
Dock process, and 10FPS still looks OK.
WIP OSX profiling
Document more findings about performance

And how we can improve it.
Re-think redraws

Draw most things in a tile-sized buffer and finish off by scaling up to
com.apple.dock.extra size.