Compensate beat note length when stretching (LMMS#5515)

* Compensate beat note length when stretching

We allow stretching beat notes to normal notes but the length starts
from -192 so there is a lag for one whole note before any change is
seen. Compensate by setting the oldNote value to 1 when stretching
if the note is 0 or below in length.

Co-authored-by: Spekular <Spekular@users.noreply.github.com>

src/gui/editors/PianoRoll.cpp

@@ -1606,6 +1606,11 @@ void PianoRoll::mousePressEvent(QMouseEvent * me )
 					// then resize the note
 					m_action = ActionResizeNote;
 
+					for (Note *note : getSelectedNotes())
+					{
+					    if (note->oldLength() <= 0) { note->setOldLength(4); }
+					}
+
 					// set resize-cursor
 					QCursor c( Qt::SizeHorCursor );
 					QApplication::setOverrideCursor( c );