larger seq cache

Make the seq cache 50% larger to allow for rounding with some image
widths, tile sizes and thread numbers.

See https://github.com/libvips/libvips/issues/1494#issuecomment-567190830

libvips/conversion/tilecache.c

@@ -937,8 +937,7 @@ vips_line_cache_build( VipsObject *object )
 	block_cache->tile_width = block_cache->in->Xsize;
 
 	/* Output has two buffers n_lines height, so 2 * n_lines is the maximum
-	 * non-locality from threading. Add another n_lines for conv / reducev
-	 * etc. 
+	 * non-locality from threading. Double again for conv, rounding, etc. 
 	 *
 	 * tile_height can be huge for things like tiff read, where we can
 	 * have a whole strip in a single tile ... we still need to have a
@@ -946,7 +945,7 @@ vips_line_cache_build( VipsObject *object )
 	 * tile boundary.
 	 */
 	block_cache->max_tiles = VIPS_MAX( 2, 
-		3 * n_lines / block_cache->tile_height );
+		4 * n_lines / block_cache->tile_height );
 
 	VIPS_DEBUG_MSG( "vips_line_cache_build: n_lines = %d\n", 
 		n_lines );

libvips/iofuncs/sinkdisc.c

@@ -254,7 +254,7 @@ wbuffer_flush( Write *write )
 	if( write->buf->area.top > 0 ) {
 		vips_semaphore_down( &write->buf_back->done );
 
-		/* Previous write suceeded?
+		/* Previous write succeeded?
 		 */
 		if( write->buf_back->write_errno ) {
 			vips_error_system( write->buf_back->write_errno,