Texture rendering over terrain glitch

Hi all,

After upgrading Cesium from 1.31 to 1.32 I have found strange glitch when rendering aerial mapping with terrain.

This is just an example, but this distortions are distributed all over the map, and are quite easy to notice.

I found that it was introduced in:
https://github.com/AnalyticalGraphicsInc/cesium/commit/f32f814c30d24cea933b7650927141738b5c5fe4 "Add flanges to heightmap tessellator." And when I'm using "createVerticesFromHeightmap.js" worker from 1.31 everything looks fine.

It seems to work fine with Cesium terrain servers, but I am using custom built terrain server.
The imagery layer is standard Bing provider.

The terrain tails has been made using this tool:
https://github.com/geo-data/cesium-terrain-builder

To reproduce please connect to this terrain tail server:
https://osmaps.ordnancesurvey.co.uk/tilesets/default/
( For Chrome you might want to use CORS extension to be able to connect, for example:
https://chrome.google.com/webstore/detail/allow-control-allow-origi/nlfbmbojpeacfghkpbjhddihlkkiljbi )

Hope this will be useful,
Best regards.

Reproduce the distortion with the linked custom terrain server and Bing imagery, then compare the 1.32 behavior with the 1.31 createVerticesFromHeightmap.js worker and the referenced heightmap tessellator change. Trace the terrain vertex generation around that entry point; done means the aerial imagery renders without the reported distortions on custom terrain while Cesium terrain still works.

Written by the indexing model from the issue text.