fix #304880: wrong beam positions when changing spatium

[Jojo-Schmitz]

for custom positioned beams of down stem notes.
Resolves https://musescore.org/en/node/304880

[Jojo-Schmitz]

Apparently this change helps in that one particular case, but I have no idea which cases it may break...

That previous construct (_direction == Direction::AUTO || _direction == Direction::DOWN) ? 0 : 1 is used in that file in multiple places, not sure it is the right thing to do there, nor whether the change here is correct.