Fix #298147 MusicXML: Handle measure number offsets

Previously it was assumed that measure numbers always started at 1 and
increased by 1 for each regular measure. Now the actual measure numbers
given in the MusicXML are used within MuseScore during import to
calculate measure number offsets. During export, these offsets are
converted back into absolute numbers for use in the MusicXML.

Backport of musescore#7617, resp. duplicate of musescore#7870 part 1, culprit for quite a lot
mtest failures

importexport/musicxml/exportxml.cpp

@@ -6130,6 +6130,7 @@ void MeasureNumberStateHandler::updateForMeasure(const Measure* const m)
             }
 
       // update measure numbers and cache result
+      _measureNo += m->noOffset();
       _cachedAttributes = " number=";
       if ((_irregularMeasureNo + _measureNo) == 2 && m->irregular()) {
             _cachedAttributes += "\"0\" implicit=\"yes\"";

importexport/musicxml/importmxmlpass2.cpp

@@ -1969,8 +1969,8 @@ void MusicXMLParserPass2::measure(const QString& partId,
                                   const Fraction time)
       {
       Q_ASSERT(_e.isStartElement() && _e.name() == "measure");
-      //QString number = _e.attributes().value("number").toString();
-      //qDebug("measure %s start", qPrintable(number));
+      int number = _e.attributes().value("number").toInt();
+      //qDebug("measure %d start", number);
 
       Measure* measure = findMeasure(_score, time);
       if (!measure) {
@@ -1979,6 +1979,8 @@ void MusicXMLParserPass2::measure(const QString& partId,
             return;
             }
 
+      measure->setNoOffset(measure->no() - number);
+
       // handle implicit measure
       if (_e.attributes().value("implicit") == "yes")
             measure->setIrregular(true);