Prevent duplicate render requests of partials

[Danielku15]

Issues

Fixes #1147

Proposed changes

Prevents requesting the rendering of the same partial multiple times. If the same partial is requested to be rendered multiple times this messes with the positioning of ties.

iOS can sometimes signal the visibility change of the same element multiple times. If this happens things break. By preventing multiple render requests we not only improve performance but also fix the problem with the ties.

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