fixup color framework

src/main/java/eu/jonahbauer/raytracing/render/spectral/SampledWavelengths.java

@@ -58,12 +58,12 @@ public final class SampledWavelengths {
      * Terminates the secondary wavelengths. This method should be called after a wavelength-dependent operation like
      * refraction that makes it incorrect to track multiple wavelengths together.
      */
-    public @NotNull SampledWavelengths collapse() {
-        if (pdf.length < 2 || pdf[1] == 0) return this;
-        var newPdf = Arrays.copyOf(pdf, pdf.length);
-        Arrays.fill(newPdf, 1, newPdf.length, 0d);
-        newPdf[0] /= newPdf.length;
-        return new SampledWavelengths(lambdas, newPdf);
+    public double collapse() {
+        if (pdf.length >= 2 || pdf[1] != 0) {
+            Arrays.fill(pdf, 1, pdf.length, 0d);
+            pdf[0] /= pdf.length;
+        }
+        return lambdas[0];
     }
 
     /*