Fix #21110 - Wavy glissando text position

Fix #21110 - Wavy glissando text position

- Text over a glissando wavy line collided with line.
- The wavy line occasionally collided with final note and was not always centred within the two notes.

Fixed in Glissando::draw() by:

- linking text vert. offset to line type
- always rounding down the number of wavy elements and positioning the line according to the number of actual elements