Auto render loop references old Cesium private variables

[schmidtk]

If the auto render loop is enabled, it will constantly throw errors due to referencing surface._tileLoadQueue, which no longer exists in Cesium.

To reproduce, load the ol3cesium example, then start the render loop:

ol3d.enableAutoRenderLoop()

Break on caught exceptions and you'll see the problem. It looks like there is now a _tileLoadQueue[High|Medium|Low], so it will hopefully be as simple as checking all three.

[gberaudo]

Thanks for the report. Are you willing to create a PR with the fix?

[schmidtk]

I probably won't get to it until next week given my current work load, but I could submit a PR for it then.

[schmidtk]

@gberaudo Sorry for the delay - had some app release work to finish first. PR is ready when you have a moment to review.