Eliminate polygon holes

Fix bug where `getMutuallyVisibleVertexIndex` in `PolygonPipeline` could return an index of `-1`. The intersection point could equal a point in the list's x and y properties but `indexOf` would return `-1` because the references were not equal.

See the second code snippet from: https://groups.google.com/forum/#!topic/cesium-dev/VbSm-c9ojso