Avoid code duplication in copy image data routine

Author: blikblum

lib/image/png.js

@@ -118,23 +118,15 @@ class PNGImage {
 
       let i = p = a = 0;
       const len = pixels.length;
-      if (this.image.bits === 8) {
-        while (i < len) {
-          for (let colorIndex = 0; colorIndex < colorCount; colorIndex++) {
-            imgData[p++] = pixels[i++];
-          }
-          alphaChannel[a++] = pixels[i++];
-        }
-      } else {
-        // copy only most significant byte (MSB) - PNG data is always stored in network byte order (MSB first)
-        while (i < len) {
-          for (let colorIndex = 0; colorIndex < colorCount; colorIndex++) {            
-            imgData[p++] = pixels[i++];
-            i++;
-          }
-          alphaChannel[a++] = pixels[i++];
-          i++;
+      // For 16bit images copy only most significant byte (MSB) - PNG data is always stored in network byte order (MSB first)
+      const skipByteCount = this.image.bits === 16 ? 1 : 0;
+      while (i < len) {
+        for (let colorIndex = 0; colorIndex < colorCount; colorIndex++) {
+          imgData[p++] = pixels[i++];
+          i += skipByteCount;
         }
+        alphaChannel[a++] = pixels[i++];
+        i += skipByteCount;
       }
 
       this.imgData = zlib.deflateSync(imgData);