update the Unfilter API in dsp to process one row independently

This will allow to work in-place on cropped area later.

Also sped up the inverse gradient filtering in SSE2 (~4%)

Change-Id: I463149eee95d36984328f163a1e17f8cabd87441

src/dec/vp8i.h

@@ -261,6 +261,7 @@ struct VP8Decoder {
   int is_alpha_decoded_;      // true if alpha_data_ is decoded in alpha_plane_
   uint8_t* alpha_plane_mem_;  // memory allocated for alpha_plane_
   uint8_t* alpha_plane_;      // output. Persistent, contains the whole data.
+  const uint8_t* alpha_prev_line_;  // last decoded alpha row (or NULL)
   int alpha_dithering_;       // derived from decoding options (0=off, 100=full)
 };