Fix findNoteAtPos logic

[gallegretti]

Issues

Fixes #744

Proposed changes

Fixes logic on findNoteAtPos to correctly find the expected note. Here is a simple codepen demonstrating the fix:
https://codepen.io/gallegretti/pen/KKyqypL
As you click on the notes, the ids will be logged on the console

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• Existing builds tests pass
• New tests were added

Further details

• This is a breaking change
• This change will require update of the documentation/website

[Danielku15]

Thanks for your contribution.