Performance regression in Entity billboards when using Resource instances

If `BillboardGraphics.image` is set to a `Resource` instance, then we end up creating a new clone of the instance every frame, this is most likely a major performance bottleneck in certain use cases.

https://github.com/AnalyticalGraphicsInc/cesium/blob/bd3c32398f8a3d95a9436ddf733ad4793063de9c/Source/DataSources/BillboardVisualizer.js#L115

Since `BillboardGraphics.image` can be a Uri, Resource, Canvas, or Image, it only makes sense to Clone the result if it's a Resource, but you don't really know that until you get the value. It probably not too much work to figure out a decent fit, but we should also track down other possible performance issues like this in the entity layer due to the Resource refactor.

Start at Source/DataSources/BillboardVisualizer.js around line 115 and trace how BillboardGraphics.image values are handled when they are Resource instances. Profile or inspect the Entity billboard update path to confirm whether a Resource is cloned every frame, then determine whether related Entity-layer performance issues are in scope. Done means eliminating the unnecessary per-frame Resource clone without changing support for Uri, Canvas, or Image values.

Written by the indexing model from the issue text.