Fix #311792: Override alter tag with note tuning value if present

Duplicate of musescore#6693

importexport/musicxml/exportxml.cpp

@@ -3305,11 +3305,17 @@ static void writePitch(XmlWriter& xml, const Note* const note, const bool useDru
                   default:                                             break;
                   }
             }
-      if (alter && !alter2)
-            xml.tag("alter", alter);
-      if (!alter && alter2)
-            xml.tag("alter", alter2);
-      // TODO what if both alter and alter2 are present? For Example: playing with transposing instruments
+      // Override accidental with explicit note tuning
+      auto tuning = note->tuning();
+      if (fabs(tuning) > 0.01)
+            alter2 = tuning / 100.0;
+      // `alter` represents the "regular" (Western) pitch which can be
+      // 0 (natural), 1 (sharp), -1 (flat), etc. or some other integer depending on transposing instruments.
+      // `alter2` represents a microtone or manually-adjusted note tuning.
+      // In MusicXML, These two values are merged in the same "alter" tag.
+      // https://usermanuals.musicxml.com/MusicXML/Content/EL-MusicXML-alter.htm
+      if (alter || alter2)
+            xml.tag("alter", alter + alter2);
       xml.tag(useDrumset ? "display-octave" : "octave", octave);
       xml.etag();
       }