Allow some floating point error when intersecting ray and triangle

I don't know why this is now necessary, but we had some tests failing
on Xcode15 (CI was fine). I assume that something changed about how
AppleClang does floating point arithmetic, even though I can't find any
info on this. Anyway - allowing more floating point error works and
the tests are now fine.

lib/vecmath/include/vecmath/intersection.h

@@ -283,24 +283,24 @@ constexpr T intersect_ray_triangle(
   const auto q = cross(t, e1);
 
   const auto u = dot(q, e2) / a;
-  if (u < T(0))
+  if (u < -constants<T>::almost_zero())
   {
     return nan<T>();
   }
 
   const auto v = dot(p, t) / a;
-  if (v < T(0))
+  if (v < -constants<T>::almost_zero())
   {
     return nan<T>();
   }
 
   const auto w = dot(q, d) / a;
-  if (w < T(0))
+  if (w < -constants<T>::almost_zero())
   {
     return nan<T>();
   }
 
-  if (v + w > T(1.0))
+  if (v + w - T(1) > constants<T>::almost_zero())
   {
     return nan<T>();
   }