[MU4] Allow tremolo stem gap to reduce when trem is very small

[asattely]

The issue: Traditional tremolos had no mitigation for becoming extremely small (and in some cases negatively long). This PR caps it at 0.6sp until the gaps become less than 0.4sp:

[cbjeukendrup]

Actually it would be nice to have a VTest that covers this situation, for example what you show on the image in the PR description. Could you please add that?

[cbjeukendrup]

@asattely @oktophonie I noticed something about the slope of tremolos. If you draw a line along the tremolo, that line will intersect the stem, instead of touching its endpoint: (left is before this PR, right is after)

It seems not to be caused by this PR, but only becomes more apparent. Is this correct?

[oktophonie]

@cbjeukendrup Tremolos are laid out 'as before' (i.e. as in 3.6). As such, this is expected behaviour for the time being. Sadly we ran out of time before 4.0 to overhaul tremolos to behave, and be placed exactly like, all other beams; once this is done, the problem you describe (along with many others) will be corrected.