Fix update of the mask projection on a layer above a layer with transformation mask

KisMergeWalker::startTrip() was expected to skip mark as N_ABOVE_FILTHY
only the currently updated transformation mask, not all layers above.
All the higher layers should be marked as FILTHY.

BUG:388547
(cherry picked from commit a41aeb7b39f63f1e08185078f32c17cd86e91dcb)