add a -dither dithering option to the decoder

Even at high quality setting, the U/V quantizer step is limited
to 4 which can lead to banding on gradient.
This option allows to selectively apply some randomness to
potentially flattened-out U/V blocks and attenuate the banding.

This option is off by default in 'dwebp', but set to -dither 50
by default in 'vwebp'.

Note: depending on the number of blocks selectively dithered,
we can have up to a 10% slow-down in decoding speed it seems.

Change-Id: Icc2446007f33ddacb60b3a80a9e63f2d5ad162de

src/dec/idec.c

@@ -423,6 +423,7 @@ static VP8StatusCode DecodePartition0(WebPIDecoder* const idec) {
   // This change must be done before calling VP8InitFrame()
   dec->mt_method_ = VP8GetThreadMethod(params->options, NULL,
                                        io->width, io->height);
+  VP8InitDithering(params->options, dec);
   if (!CopyParts0Data(idec)) {
     return IDecError(idec, VP8_STATUS_OUT_OF_MEMORY);
   }