fix #286185: bends above the staff drawn incorrectly

The original impelementation of bends was for tablature only
and had no need to handle notes above the staff.
Eventually standard staves were supported,
but no allowances were made for notes above the staff.
This changes simply adjusts the vertical position of the bend
to be as high above notes above the staff
as they are notes on the top line.
It's just a matter of forcing the notePos (used in both layout and drawing)
to be no less than zero.

libmscore/bend.cpp

@@ -91,6 +91,7 @@ void Bend::layout()
             }
       else {
             notePos   = note->pos();
+            notePos.ry() = qMax(notePos.y(), 0.0);
             noteWidth = note->width();
             }
       QRectF bb;