Adjust ui element positioning to support browser scaling

[Danielku15]

Issues

Fixes wrong cursor placement when using browser zoom (detected as part of 1.2.3 testing)

Proposed changes

The problem with 1x1px elements being scaled using transforms is the order how the browser applies the browser zoom (not pinch zoom). First the 1x1px element is scaled to the zoom level and then the scale is applied. on a 1x1px element, it will stay a 1x1px element on the browser zoom because on those pixel sized elements no fractions are possible. Then the scale is applied keeping the element always on the 100% zoom level not matching the music sheet.

As a fix we use now a 100x100px element for cursors and selections which is then scaled correctly. This way the 100x100px element gives us enough zoom options to scale along the browser zoom.

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