Fix labeling using perimeter with repeating label distance set

If the visible part of a polygon is clipped and becomes a multipolygon, only
one label is plotted on the wrong side of the polygon.

Settings:
Placement: Using Perimeter
Allowed positions: Below line / Line orientation dependent position checked
Repeat: 100 mm

Fix #15341

(cherry-picked from c0b1684)

src/core/qgspallabeling.cpp

@@ -2517,7 +2517,8 @@ void QgsPalLayerSettings::registerFeature( QgsFeature& f, QgsRenderContext &cont
   }
 
   GEOSGeometry* geos_geom_clone;
-  if ( GEOSGeomTypeId_r( QgsGeometry::getGEOSHandler(), geos_geom ) == GEOS_POLYGON && repeatDistance > 0 && placement == Line )
+  GEOSGeomTypes geomType = (GEOSGeomTypes) GEOSGeomTypeId_r( QgsGeometry::getGEOSHandler(), geos_geom );
+  if ( (geomType == GEOS_POLYGON || geomType == GEOS_MULTIPOLYGON) && repeatDistance > 0 && placement == Line )
   {
     geos_geom_clone = GEOSBoundary_r( QgsGeometry::getGEOSHandler(), geos_geom );
   }