Artifacts in example Shaded Relief (with WebGL)

[Dolpic]

Describe the bug
In the example named "Shaded Relief (with WebGL)", when zooming on the map, lines of different color are generated at the intersection of tile.

To Reproduce
Load the example at https://openlayers.org/en/latest/examples/webgl-shaded-relief.html.
This can be seen on any data loaded with WebGL tiles where the style makes a computation requiring neighbors pixels.

Expected behavior
The tiles should appear without artifacts between them.

This bug is the cause of two issues:

1. The first is that, on a tile's borders, the slope is computed with pixels outside the tiles, which by the WebGL texture are the border pixel repeated, leading to an incorrect slope.
2. Interpolation is activated for the map, and the border of the tile are interpolated by WebGL which create further artifacts.

The PR #15478 fixes this issue.

[mike-000]

This can be fixed by using buffered tiles (as done by Mapbox). Tiles with a gutter/buffer added are available from Nextzen and Mapbox, or could be produced by combining unbuffered tiles in a custom loader, see #14964 and #13647.