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/webp.c

@@ -474,6 +474,7 @@ static VP8StatusCode DecodeInto(const uint8_t* const data, size_t data_size,
         // This change must be done before calling VP8Decode()
         dec->mt_method_ = VP8GetThreadMethod(params->options, &headers,
                                              io.width, io.height);
+        VP8InitDithering(params->options, dec);
         if (!VP8Decode(dec, &io)) {
           status = dec->status_;
         }