use DC error diffusion for U/V at low-quality

This fixes some color smearing due to heavy quantization.
This is only enabled for q <= 30 (cf ERROR_DIFFUSION_QUALITY)

Change-Id: I07e83a4d38461357a32c9e214f7eadc6db73baa9

src/enc/vp8i_enc.h

@@ -120,6 +120,9 @@ static WEBP_INLINE int QUANTDIV(uint32_t n, uint32_t iQ, uint32_t B) {
 // Uncomment the following to remove token-buffer code:
 // #define DISABLE_TOKEN_BUFFER
 
+// quality below which error-diffusion is enabled
+#define ERROR_DIFFUSION_QUALITY 30
+
 //------------------------------------------------------------------------------
 // Headers
 
@@ -201,6 +204,8 @@ typedef struct {
   score_t i4_penalty_;   // penalty for using Intra4
 } VP8SegmentInfo;
 
+typedef int16_t DError[2 /* u/v */][2 /* top or left */];
+
 // Handy transient struct to accumulate score and info during RD-optimization
 // and mode evaluation.
 typedef struct {
@@ -213,6 +218,7 @@ typedef struct {
   uint8_t modes_i4[16];       // mode numbers for intra4 predictions
   int mode_uv;                // mode number of chroma prediction
   uint32_t nz;                // non-zero blocks
+  int16_t derr[2][3];         // DC diffusion errors for U/V for blocks #1/2/3
 } VP8ModeScore;
 
 // Iterator structure to iterate through macroblocks, pointing to the
@@ -242,6 +248,9 @@ typedef struct {
   int           count_down0_;      // starting counter value (for progress)
   int           percent0_;         // saved initial progress percent
 
+  DError        left_derr_;        // left error diffusion (u/v)
+  DError       *top_derr_;         // top diffusion error - NULL if disabled
+
   uint8_t* y_left_;    // left luma samples (addressable from index -1 to 15).
   uint8_t* u_left_;    // left u samples (addressable from index -1 to 7)
   uint8_t* v_left_;    // left v samples (addressable from index -1 to 7)
@@ -401,6 +410,7 @@ struct VP8Encoder {
   uint8_t*   uv_top_;    // top u/v samples.
                          // U and V are packed into 16 bytes (8 U + 8 V)
   LFStats*   lf_stats_;  // autofilter stats (if NULL, autofilter is off)
+  DError*    top_derr_;  // diffusion error (NULL if disabled)
 };
 
 //------------------------------------------------------------------------------