[trunk] editing grace note beams corrupts stem layout

Well, see the picture:

What I've done:
1) Double click the beam
2) Moving the left square up
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Notice that this only happens if there are 3 or more grace notes. 1 or 2 grace notes work well.
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I've tagged this as critical because it corrupts the notation, and in some cases editing the beam is mandatory because of note crashes, as in this other bug: http://musescore.org/en/node/14774
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BTW in 1.1. it works well.
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r.5264 windows xp