Polygons disappear at close zoom levels

Load [this Sandcastle](https://sandcastle.cesium.com/#c=rZJfa4MwFMW/SvClCi7Wyii0tmy4x8EeBnuZe0j1VsNiIjfR4ka/+6J2Rbq9jO485Z6c+7vkT6akNqTlcAAkGyLhQBLQvKnoy+C5s2woEyUN4xJw5q1TOeYpSMMNB01ZnrufqSRWtRJdoSRZkZPRqwRelGZF5v7E44AMs7Kz0dPIhKGxKyYjukdVPUCBAPoekXXu600Y3tK5T6IlnWL+qjNmcSVmvhww0bWYxYAJ/wdjj/bmjZxjKo/e2vGdWJtOwPabfserWqEhDQqX0sBAVQtmrz3YNdk7GJpp3T9xH42DaWuc85bwfJM6F18idUgmmNZ2Z98I8cw/IHW2cWDzP1qFYjmXxVMLKFjXx8pw+zialNI4sOXvnUYpsWN4Qf4C) and keep zooming in, eventually you'll hit the ground or near the ground and the polygon will disappear. You can even tilt the camera and view the horizon and the polygon doesn't reappear. Is this is tessellation issue? Or some sort of frustum bug? CC @bagnell

Start by loading the linked Sandcastle reproduction and repeatedly zooming toward the ground, then tilt toward the horizon to confirm whether the polygon remains absent. Investigate the polygon rendering path to determine whether tessellation or frustum handling causes the disappearance; done means identifying and correcting the condition so the polygon remains visible at these close zoom levels.

Written by the indexing model from the issue text.