Fix DynamicPathVisualizer handling of ReferenceProperty

`DynamicPathVisualizer` was relying on a hidden `_referenceFrame` property on position objects.  This is not the case with `ReferenceProperty`.  This changes it to use the public API.

Also, the subsampling code for paths was not unwrapping `ReferenceProperty` instances, causing it to fallback to generic sub-optimal sampling.  This addresses that as well and adds tests to verify it.
`@Type` -> `@type`
Merge pull request #1835 from AnalyticalGraphicsInc/dynamicPath-referenceProperty

Fix DynamicPathVisualizer handling of ReferenceProperty
Merge remote-tracking branch 'origin/master' into julianDate-require-result

Conflicts:
	Specs/DynamicScene/DynamicPathVisualizerSpec.js