fix: ensure clean beam-stem alignment on subpixel rendering

[Danielku15]

Issues

Fixes #2487

Proposed changes

Seems this problems was actually in since a while (already released). Due to some rounding "optimizations" the beams turned out wobbly.

We now do rounding at alternative places to avoid a wobbly slant:

1. We avoid subpixel start-end coordinates for the beams.
2. We round the x-coordinate for the beam start and end to align it with the next full-pixel of the stem.

This way we have a clean slant in most scenarios. The only exception is when the stem is wide enough to have a visual horizontal line. This in this case we have a bit of a slope-plane-slope effect.

To avoid this alphaTab would need to bigger rework the beam drawing by drawing the beams as far as possible with 1 beam instead a beat-to-beat drawing as it is today. We can improve this if it really becomes a problem for someone.

Checklist

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

Further details

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