Bare min. from musescore#9068 to be able to port musescore#9137 to 3.x

(Very partial) backport of musescore#9068, the rest is too large for too litle
benefit (mainly due to the missing UI) and also requires C++17.

libmscore/measure.cpp

@@ -4596,4 +4596,26 @@ void Measure::computeMinWidth()
       computeMinWidth(s, x, isSystemHeader);
       }
 
+qreal Measure::computeFirstSegmentXPosition(Segment* segment)
+      {
+      qreal x = 0;
+
+      Shape ls(QRectF(0.0, 0.0, 0.0, spatium() * 4));
+
+      x = segment->minLeft(ls);
+
+      if (segment->isChordRestType())
+            x += score()->styleP(hasAccidental(segment) ? Sid::barAccidentalDistance : Sid::barNoteDistance);
+      else if (segment->isClefType() || segment->isHeaderClefType())
+            x += score()->styleP(Sid::clefLeftMargin);
+      else if (segment->isKeySigType())
+            x = qMax(x, score()->styleP(Sid::keysigLeftMargin));
+      else if (segment->isTimeSigType())
+            x = qMax(x, score()->styleP(Sid::timesigLeftMargin));
+
+      x += segment->extraLeadingSpace().val() * spatium();
+
+      return x;
+      }
+
 }

libmscore/measure.h

@@ -279,6 +279,9 @@ class Measure final : public MeasureBase {
       void checkTrailer();
       void setStretchedWidth(qreal);
       void layoutStaffLines();
+
+      qreal computeFirstSegmentXPosition(Segment* segment);
+
       };
 
 }     // namespace Ms