swscale: fix ticket #4881

When scaling only a slice of a frame the output was written always
in the first lines leaving the rest of the frame black.

libswscale/swscale_internal.h

@@ -1013,7 +1013,8 @@ typedef struct VScalerContext
 } VScalerContext;
 
 // warp input lines in the form (src + width*i + j) to slice format (line[i][j])
-int ff_init_slice_from_src(SwsSlice * s, uint8_t *src[4], int stride[4], int srcW, int lumY, int lumH, int chrY, int chrH);
+// relative=true means first line src[x][0] otherwise first line is src[x][lum/crh Y]
+int ff_init_slice_from_src(SwsSlice * s, uint8_t *src[4], int stride[4], int srcW, int lumY, int lumH, int chrY, int chrH, int relative);
 
 // Initialize scaler filter descriptor chain
 int ff_init_filters(SwsContext *c);