Made imagerySplitPosition use absolute coordinates

[jasonbeverage]

This should fix #5106 by making the imagerySplitPosition option use absolute coordinates instead of relative coordinates so that the multiple viewport rendering of the 2D view won't cause issues.

I've also updated the sandcastle example to use absolute coordinates and put a prerender listener in there to make sure the split position is correct when the window is resized.

[pjcozzi]

Thanks @jasonbeverage! Looks good at quick glance. Can you please update CHANGES.md, including a link to #5106?

@rahwang can you please review and test this?

[pjcozzi]

Ah, this is a breaking change, right? Can you please make a new property name and deprecate the old one? See https://github.com/AnalyticalGraphicsInc/cesium/tree/master/Documentation/Contributors/CodingGuide#deprecation-and-breaking-changes

[jasonbeverage]

Hey @pjcozzi , I opened up another PR at #5151 that still uses the relative coordinates publicly and computes the absolute coordinates in UniformState.update. This avoids having to deprecate the property and also makes it easier for users to deal with the canvas resizing. I'm going to close this PR in favor of the other one if that is ok with you.

[pjcozzi]

Sounds good, thanks @jasonbeverage!