Fix horizon culling causing terrain tiles to disappear

[SamHL]

Visual symptoms: - At certain camera orientations, large portions of the visible Earth (often an entire hemisphere) would suddenly disappear - The issue was orientation-sensitive: a small rotation of the camera would cause the missing terrain to reappear - Most noticeable when viewing the globe from orbital distances

Root cause:
The horizon culling in SurfaceNode::isVisible() was calling Horizon::isVisible() with radius=0, treating each tile's horizon culling point as an infinitesimal point rather than accounting for the tile's actual spatial extent. This caused tiles near the horizon boundary to be incorrectly culled even when part of the tile should have been visible.

Additionally, Horizon::isVisible() had a potential numerical precision issue where the calculation sqrt(dot(VT,VT) - a*a) could produce NaN if floating-point errors caused the value under the sqrt to go negative.

The fix:

1. SurfaceNode.h: Pass the tile's bounding sphere radius to the horizon visibility test, making culling more conservative by accounting for the tile's actual size

2. Horizon.cpp: Add numerical safeguards to prevent NaN propagation:

   • Clamp cSquared to >= 0 before taking sqrt
   • If the final result is NaN, default to treating tile as visible

[gwaldron]

Hi Sam and thanks for the submission.

I haven't seen this problem; do you have steps to reproduce it?
Do your numerical fixes in Horizon.cpp make the problem go away by themselves?

I ask because the whole idea of the "horizon culling point" is to be a single point (without any radius) that can determine the visibility of the entire tile. Here's a reference article:
https://cesium.com/blog/2013/05/09/computing-the-horizon-occlusion-point/

It is definitely possible we are not calculating the point correctly and that is causing artifacts (though I have not seen this problem personally).