Regression fix for alpha unfiltering:

For vertical filter, correctly initialize 'preds' when not starting at
row 0.

Change-Id: I69697d73dde8a2f701f376ff3986d09fa6f7fee3

src/utils/filters.c

@@ -112,6 +112,9 @@ static WEBP_INLINE void DoVerticalFilter(const uint8_t* in,
     row = 1;
     in += stride;
     out += stride;
+  } else {
+    // We are starting from in-between. Make sure 'preds' points to prev row.
+    preds -= stride;
   }
 
   // Filter line-by-line.