Depth fail epsilon

Cesium has a mechanism to draw primitives that fails the depth test with a `depthFailAppearance`, but if a primitive is coplanar with other geometry there may be flickering between the `appearance` and the `depthFailAppearance`. This is a screenshot of a polygon on a point cloud.

It would be useful to have a depth fail epsilon where the `depthFailAppearance` is only used if the rendered geometry fails the depth test past by some epsilon value in meters. This would help stop the flickering when viewing the primitive face on but still use the `depthFailAppearance` correctly when viewing it from the other side where the geometry is obscured.

There was also some discussion about this for point clouds in https://github.com/AnalyticalGraphicsInc/cesium/issues/6714#issuecomment-399439401

No file or test is named. Start by locating the Cesium primitive rendering path that handles depthFailAppearance, then read the discussion in issue #6714 about point clouds. Done means preventing coplanar flickering with a configurable depth-fail epsilon while preserving depth-fail rendering when geometry is obscured.

Written by the indexing model from the issue text.