add a 'last_y' field to WebPDecParams

this allows to keep an exact track of the last displayable pixel row

Change-Id: I590306735a82b9249e7bc9fe64eeb3e5bea3376b

src/dec/webpi.h

@@ -31,6 +31,7 @@ typedef struct {
   int u_stride;
   int v_stride;
   WEBP_CSP_MODE mode;
+  int last_y;           // coordinate of the line that was last output
 } WebPDecParams;
 
 // If a RIFF container is detected, validate it and skip over it. Returns