Entity primitive continuously re-drawn when position is SampledProperty

[thw0rted]

Example in Sandcastle, based on the Interpolation demo.

Note that the debug frame counter shows continuous frames being drawn even though we're in requestRenderMode = true and the animation is paused. I found this because my app is rendering with a poor framerate in some cases, and I tracked it to a single rogue entity that has a SampledPositionProperty and an ellipsoid graphic. I dug in the debugger a little and figured out that for some reason, the associated primitive collection is being updated just about every frame, which didn't make sense to me as I wasn't modifying anything.

This doesn't happen in 3D scene mode, and it also doesn't happen with other 2D primitives (billboard, label, etc). I haven't tried with other solids but I'd be surprised if the issue was limited to ellipsoid.