Decouple movie tracing through metric slabs

tests/test_geodesic.c

@@ -27,8 +27,14 @@ static int check_ray(const SpacetimeSource *source,
 
 int main(void) {
   SpacetimeSource source = {0};
+  MetricSlab *slab = NULL;
+  MetricData metric;
   const ObserverState observer = observer_fixed_at_origin();
-  if (spacetime_create_minkowski(&source, 10.0))
+  if (spacetime_create_minkowski(&source, 10.0) ||
+      spacetime_load_slab(&source, 0.0, -1.0, &slab) ||
+      spacetime_slab_eval(slab, -0.5, (double[]){0.0, 0.0, 0.0}, &metric) ||
+      metric.alpha != 1.0 ||
+      !spacetime_slab_eval(slab, 0.25, (double[]){0.0, 0.0, 0.0}, &metric))
     return 1;
   int result = check_ray(&source, &observer, (double[]){1.0, 0.0, 0.0},
                          (double[]){0.0, 0.0, -1.0}) ||
@@ -57,6 +63,7 @@ int main(void) {
     }
   }
   observer_track_destroy(&accelerated);
+  spacetime_free_slab(slab);
   spacetime_destroy(&source);
   return result;
 }