Polyline not hidden when below earth's surface

[GatorScott]

Duplicate by modifying the longitude of the "blue" line's first coordinate in the Polyline Sandcastle example to be 170.0. Reference documentation doesn't specify expected behavior.

[mramato]

Because we do not yet support vector data on terrain, we default scene.globe.depthTestAgainstTerrain to false; which causes all primitives to get drawn on top of the terrain even if they are under the surface of the earth. This is so items that are are on the earth (lines, billboards, polygons) look good and do not suffer from z-fighting. Settings scene.globe.depthTestAgainstTerrain to true will cause primitives to be depth tested and hidden when under terrain, but polygons and lines on the surface will have really bad visual artifacts (and billboards/labels can be partially buried in terrain).

When we have terrain draping support for all primitives; we'll be able toggle the flag without loss of visual quality.

@pjcozzi or @kring can hopefully chime in here if I got anything wrong here.

[pjcozzi]

@mramato is right. Keep an eye on #2172.