Math: Fix Math::Intersection::sphereFrustum

[Squareys]

Hi @mosra !

As per #453 (Fix sphere frustum intersection), here's the fix plus extensive tests.

I made a conscious descision to mark "touching" geometries as non-intersection. Even though mathematically, the volume is not empty, it's infinitely small and hence useless for the main use case, which will be frustum culling.

Best, Jonathan

[codecov[bot]]

Codecov Report

Merging #482 (47e7f6b) into master (6f1f50d) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   77.12%   77.12%   -0.01%     
==========================================
  Files         415      415              
  Lines       25498    25497       -1     
==========================================
- Hits        19665    19664       -1     
  Misses       5833     5833              

Impacted Files	Coverage Δ
src/Magnum/Math/Intersection.h	100.00% <100.00%> (ø)

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