Rework grace beat positioning

[Danielku15]

Description

This issue is a follow up of #227

Today alphaTab attempts to position grace beats according to the time they are played. Due to various timing constraints not the actual beat duration is considered for this positioning, but 1/4 of the previous beat duration is used. If we use the actual beat time, it can happen that the gracebeat is positioned on top of the previous one:

Problem with grace beat stealing full duration of previous beat 👎

Only stealing 1/4 of the previous beat per grace note 👍

This 1/4 rule bypasses the issue of the screenshot above but it is not really correct in respect of the actual timing. This can be seen well when we have multiple grace notes and combinations. The positioning does not really make sense.

So the overall grace note positioning system is not really doing well and should be reworked.

Possible Solutions

GuitarPro solves this issue simply by not positioning the beats in respect to their playback time. They simply stick right before the next note being played.

Default

Manually scaled

[Danielku15]

Grace beats are not sticking to their next non-grace beat if possible. For incomplete graces we just put them after the last proper beat. This rework also improved some timing related positioning when it comes to grace notes. Some combinations are now more compact, others need a bit more space.
Comparison: