Regression: harsh attack sound in vibraphone

Windows 10, GIT commit: 871c8ce

See https://musescore.org/en/node/197131#comment-748491 and followup for more discussion. Also original report in https://musescore.org/en/node/207376#comment-748311.

It appears that something changed between 2.0.3 and 2.1 in our fluidsynth implementation to adversely affect the sound of vibraphone from the default soundfont. Perhaps other instruments as well. This is a separate issue from the loud pops that have been reported multiple times. What I am talking about here is a subtler but still objectionable change (and it is not fixed by the proposed fix for #197131: Regression: Bass synthesizer and bass guitar create popping sounds.

here is how to reproduce the problem:

1) new score for vibraphone
2) enter a single note (middle line B, for instance)

If you listen to the playback of that note, it sounds "normal" - same as in 2.0.3. Also if you exit note input mode and select the note, it plays back "normally" as well. But if you actually play the score, there is a noticeable "clacking" sound to the attack that isn't there when entering or selecting the note. And it wasn't that way for 2.0.3. To hear the effect more strongly, enter a full measure of sixteenths and play that.

It is at least *possible* that this is actually correct, and that the sample was recorded with a very hard mallet but somehow 2.0.3 was not reproducing this accurately. I suspect not, however.