Add static rendering option

We should support static rendering (don't redraw if nothing has changed)

We can either place all the necessary logic within the library and
check if anything has changed, redraw if necessary. This places the
burden on us.

Or we can provide an option for users of the library to set
if they want static rendering. This will save us writing the
code at the expense of having the user to toggle it. Of course
the users would know if their scene should be rendered statically.

Either case, we should provide one of these choices. When we
do, we need to figure out what to do with FPS. Perhaps add
documentation somewhere which states it is only relevant
when frames are being redrawn? Maybe change the getFPS() function
to getLastFPS()?

I think this would be great for mobile devices (once WebGL lands)
or on any device where FPS is low.