Make notes scale from the center when using the mouse

[udf]

When holding down shift and dragging the mouse, you can scale a note. Previously it scaled about the original mouse location, which resulted in unintuitive scaling behaviour where the note moves while being scaled. This change brings it in line with scaling via the keyboard (shift+arrow keys).

Old:

https://github.com/rr-/szurubooru/assets/13610073/25106906-42cb-487f-955d-4bd1762cf887

New:

https://github.com/rr-/szurubooru/assets/13610073/28bd06ac-cee6-4574-8c9d-fd6d211d6faa