Improved alpha cleanup for the webp encoder when prediction transform is used.

Gives 0.9% smaller (2.4% compared to before alpha cleanup) size on the 1000 PNGs dataset:
Alpha cleanup before: 18856614
Alpha cleanup after: 18685802
For reference, with no alpha cleanup: 19159992

Note: WebPCleanupTransparentArea is still also called in WebPEncode. This cleanup still helps
preprocessing in the encoder, and the cases when the prediction transform is not used.

Change-Id: I63e69f48af6ddeb9804e2e603c59dde2718c6c28

src/enc/alpha.c

@@ -67,6 +67,11 @@ static int EncodeLossless(const uint8_t* const data, int width, int height,
 
   WebPConfigInit(&config);
   config.lossless = 1;
+  // Enable exact, or it would alter RGB values of transparent alpha, which is
+  // normally OK but not here since we are not encoding the input image but  an
+  // internal encoding-related image containing necessary exact information in
+  // RGB channels.
+  config.exact = 1;
   config.method = effort_level;  // impact is very small
   // Set a low default quality for encoding alpha. Ensure that Alpha quality at
   // lower methods (3 and below) is less than the threshold for triggering

src/enc/vp8l.c

@@ -1012,7 +1012,8 @@ static WebPEncodingError ApplyPredictFilter(const VP8LEncoder* const enc,
   const int transform_height = VP8LSubSampleSize(height, pred_bits);
 
   VP8LResidualImage(width, height, pred_bits, low_effort, enc->argb_,
-                    enc->argb_scratch_, enc->transform_data_);
+                    enc->argb_scratch_, enc->transform_data_,
+                    enc->config_->exact);
   VP8LPutBits(bw, TRANSFORM_PRESENT, 1);
   VP8LPutBits(bw, PREDICTOR_TRANSFORM, 2);
   assert(pred_bits >= 2);