Observer: parameterize Schwarzschild camera view

tests/test_schwarzschild.c

@@ -7,6 +7,7 @@ int main(void) {
   SpacetimeSource spacetime = {0};
   MetricData metric;
   ObserverState observer;
+  ObserverState oriented_observer;
   ObserverState inward_observer;
   const GeodesicTraceConfig trace = {.coordinate_time_step = 0.1,
                                      .max_steps = 4096,
@@ -17,6 +18,16 @@ int main(void) {
       !isfinite(metric.alpha) || !isfinite(metric.gamma[0][0]) ||
       !isfinite(metric.K[0][0]) ||
       observer_static_schwarzschild_ks(1.0, 30.0, &observer) ||
+      observer_static_schwarzschild_ks_look_at(1.0, 40.0, 270.0, 30.0,
+                                                &oriented_observer) ||
+      fabs(oriented_observer.coordinate_position[0]) > 1e-12 ||
+      fabs(oriented_observer.coordinate_position[1] + 20.0) > 1e-12 ||
+      fabs(oriented_observer.coordinate_position[2] - 20.0 * sqrt(3.0)) >
+          1e-12 ||
+      fabs(oriented_observer.tetrad[1][1]) > 1e-12 ||
+      fabs(oriented_observer.tetrad[1][2] - 0.5 * sqrt(0.95)) > 1e-12 ||
+      fabs(oriented_observer.tetrad[1][3] + sqrt(0.95) * sqrt(3.0) / 2.0) >
+          1e-12 ||
       observer_inward_schwarzschild_ks(1.0, 30.0, 0.5,
                                        &inward_observer) ||
       fabs(inward_observer.tetrad[0][0] -