Use deterministic random-dithering during RGB->YUV conversion

-> helps debanding (sky, gradients, etc.)

This dithering can only be triggered when using -preset photo
or -pre 2 (as a preprocessing). Everything is unchanged otherwise.

Note that this change is likely to make the perceived PSNR/SSIM drop
since we're altering the input internally.

Change-Id: Id8d4326245d9b828141de162c94ba381b1fa5813

src/enc/webpenc.c

@@ -358,7 +358,17 @@ int WebPEncode(const WebPConfig* config, WebPPicture* pic) {
     VP8Encoder* enc = NULL;
     if (pic->y == NULL || pic->u == NULL || pic->v == NULL) {
       // Make sure we have YUVA samples.
-      if (!WebPPictureARGBToYUVA(pic, WEBP_YUV420)) return 0;
+      float dithering = 0.f;
+      if (config->preprocessing & 2) {
+        const float x = config->quality / 100.f;
+        const float x2 = x * x;
+        // slowly decreasing from max dithering at low quality (q->0)
+        // to 0.5 dithering amplitude at high quality (q->100)
+        dithering = 1.0f + (0.5f - 1.0f) * x2 * x2;
+      }
+      if (!WebPPictureARGBToYUVADithered(pic, WEBP_YUV420, dithering)) {
+        return 0;
+      }
     }
 
     enc = InitVP8Encoder(config, pic);