put the shrink line cache back

so we can use THINSTRIP for the vips_shrink() operation

libvips/resample/shrink.c

@@ -322,15 +322,12 @@ vips_shrink_build( VipsObject *object )
 	if( vips_image_copy_fields( resample->out, resample->in ) )
 		return( -1 );
 
-	/* We want to be able to work with sequential sources. THINSTRIP would
+	/* THINSTRIP will work, FATSTRIP will break seq mode. If you combine
-	 * be OK, but FATSTRIP would be a disaster: thread 2 would be given a
+	 * shrink with conv you'll need to use a line cache to maintain
-	 * strip some way down the output, which would be a huge distance down
+	 * sequentiality.
-	 * the input.
-	 *
-	 * Make sure we always work by insisting on SMALLTILE.
 	 */
 	vips_demand_hint( resample->out, 
-		VIPS_DEMAND_STYLE_SMALLTILE, resample->in, NULL );
+		VIPS_DEMAND_STYLE_THINSTRIP, resample->in, NULL );
 
 	/* Size output. Note: we round the output width down!
 	 */

tools/vipsthumbnail.c

@@ -162,8 +162,19 @@ shrink_factor( IMAGE *in, IMAGE *out,
 	}
 
 	/* Shrink! 
+	 *
+	 * We want to make sure we read the image sequentially.
+	 * However, the convolution we may be doing later will force us 
+	 * into SMALLTILE or maybe FATSTRIP mode and that will break
+	 * sequentiallity.
+	 *
+	 * So ... read into a cache where tiles are scanlines, and make sure
+	 * we keep enough scanlines to be able to serve a line of tiles.
 	 */
-	if( im_shrink( x, t[3], shrink, shrink ) ||
+	if( im_shrink( x, t[2], shrink, shrink ) ||
+		im_tile_cache( t[2], t[3], 
+			t[2]->Xsize, 1, 
+			VIPS__TILE_HEIGHT * 2 ) ||
 		im_affinei_all( t[3], t[4], 
 			interp, residual, 0, 0, residual, 0, 0 ) )
 		return( -1 );