fix a bug in PdfUtil#getSolidAngle

src/main/java/eu/jonahbauer/raytracing/scene/util/PdfUtil.java

@@ -13,6 +13,7 @@ public final class PdfUtil {
         var j = b.minus(o).unit();
         var k = c.minus(o).unit();
 
-        return 2 * Math.atan(Math.abs(i.times(j.cross(k))) / (1 + i.times(j) + j.times(k) + k.times(i)));
+        var angle = 2 * Math.atan(Math.abs(i.times(j.cross(k))) / (1 + i.times(j) + j.times(k) + k.times(i)));
+        return angle < 0 ? 2 * Math.PI + angle : angle;
     }
 }