Expose scene option to disable order-independent translucency

[pjcozzi]

OIT has caused problems on enough machines that we should provide an option to disable it.

https://groups.google.com/forum/#!topic/cesium-dev/AV6la4vZzwM

Since this is probably too much of a pain to do at runtime, this could be a Scene constructor option and then a @readonly property.

If we look at enough WebGL Reports, can we determine what systems this is a problem for and automatically disable it?

[pjcozzi]

@emackey can you email the forum threads (there's two or three) and let them know this will be in 1.2?