Fix render stuttering

[sunjay]

This is a "small" issue, in that I think you really have to look for this in order to notice it. On some machines, when you run a turtle program (e.g. examples/circle.rs), you can see the turtle visibly "jump around" a bit as it draws. It's almost as if an afterimage of the turtle is being shown as the new image is being drawn on top.

My guess is that this is probably caused by lag in the rendering. It's possible that I'm not swapping the buffers correctly or that vsync isn't working as I expect. It's also possible that our rendering library, pathfinder, makes some assumptions that I don't know about.

This is definitely an interesting issue to look into if you like debugging graphics stuff!