Zoom to mouse position significantly inprecise

In most situations zooming to mouse position is done not in mouse position, but in area near mouse position.
Consider zooming to and from point in Iceland here: http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Hello%20World.html&label=Showcases


IMO, mouse zooming must have invariant that Earth point under cursor is always preserved while zooming (except cases, when mouse position goes out of Earth).

Reproduce the zoom behavior using the linked Cesium Sandcastle Hello World example, focusing on zooming toward a point in Iceland. Trace the mouse-zoom interaction from that entry point and verify that the Earth point under the cursor remains fixed during zooming, except when it leaves the Earth.

Written by the indexing model from the issue text.