Fix typo in variable name - watermark is working again
(cherry picked from commit 08e6fd1140083de5fe617d0d0f3f8a70374676c6)
masks: new calling convention for dt_xyz_get_mask_roi()

output buffer needs to be supplied by the calling routine
(cherry picked from commit 5bcd5eaceb5157fcc662712ed73dfda86472361b)