Enable Near-lossless feature.

Enable the WebP near-lossless feature by pre-processing the image to smoothen
the pixels.

On a 1000 PNG image corpus, for which WebP lossless (default settings) gets
25% compression gains, following is the performance of near-lossless feature
at various '-near_lossless' levels:
-near_lossless 90: 30% (very very high PSNR 54-60dB)
-near_lossless 75: 38% (very high PSNR 48-54dB)
-near_lossless 50: 45% (high PSNR 42-48dB)
-near_lossless 25: 48% (moderate PSNR 36-42dB)
-near_lossless 10: 50% (PSNR 30-36dB)

WebP near-lossless is specifically useful for discrete-tone images like
line-art, icons etc.

Change-Id: I7d12a2c9362ccd076d09710ea05c85fa64664c38

examples/cwebp.c

@@ -615,10 +615,8 @@ static void HelpLong(void) {
          "                           green=0xe0 and blue=0xd0\n");
   printf("  -noalpha ............... discard any transparency information\n");
   printf("  -lossless .............. encode image losslessly\n");
-#ifdef WEBP_EXPERIMENTAL_FEATURES
-  printf("  -near_lossless ......... use near-lossless image\n"
+  printf("  -near_lossless <int> ... use near-lossless image\n"
          "                           preprocessing (0=off..100)\n");
-#endif
   printf("  -hint <string> ......... specify image characteristics hint,\n");
   printf("                           one of: photo, picture or graph\n");