Bring in NeuQuant as an option for GIF.

It was here originally, so we might as well have it just to compare what's different. It tends to analyze the color table well, but doesn't really dither, so maybe it could be used for an analysis step...

src/main/java/com/github/tommyettinger/anim8/AnimatedGif.java

@@ -187,6 +187,12 @@ public void write(OutputStream output, Array<Pixmap> frames, int fps) {
      */
     public boolean fastAnalysis = true;
 
+    /**
+     * Can be changed to true to use a completely different color quantization and dithering algorithm, NeuQuant.
+     * When enabled, the {@link #palette} is ignored, and so is {@link #ditherStrength}.
+     */
+    public boolean useNeuQuant = false;
+
     /**
      * Often assigned as a field, the palette can be null (which means this may analyze each frame for its palette,
      * based on the setting for {@link #fastAnalysis}), or can be an existing PaletteReducer. You may want to create a
@@ -338,7 +344,11 @@ public boolean addFrame(Pixmap im) {
             ++seq;
             image = im;
             getImagePixels(); // convert to correct format if necessary
-            analyzePixels(); // build color table & map pixels
+            // build color table & map pixels
+            if(useNeuQuant)
+                analyzePixelsNQ();
+            else
+                analyzePixels();
             if (firstFrame) {
                 writeLSD(); // logical screen descriptor
                 writePalette(); // global color table
@@ -1504,6 +1514,33 @@ protected void analyzePixels() {
         }
     }
 
+    /**
+     * Analyzes image colors and creates color map.
+     */
+    protected void analyzePixelsNQ() {
+        int nPix = width * height;
+        indexedPixels = new byte[nPix];
+        NeuQuant nq = new NeuQuant(image, 10);
+        // initialize quantizer
+        colorTab = nq.process(); // create reduced palette
+
+        for (int i = 0, c = 0; i < colorTab.length; i += 3, c++) {
+            usedEntry[c] = false;
+        }
+        // map image pixels to new palette
+        int k = 0;
+        for (int y = 0, i = 0; y < height; y++) {
+            for (int x = 0; x < width; x++) {
+                int rgba = image.getPixel(x, y);
+                int index = nq.map(rgba >>> 24, rgba >>> 16 & 255, rgba >>> 8 & 255);
+                usedEntry[index] = true;
+                indexedPixels[i++] = (byte) index;
+            }
+        }
+        colorDepth = 8;
+        palSize = 7;
+    }
+
     /**
      * Extracts image pixels into byte array "pixels"
      */