n#793999: PPTX Export - background image fit to slide.

oox/source/export/drawingml.cxx

@@ -561,6 +561,11 @@ void DrawingML::WriteBlipMode( Reference< XPropertySet > rXPropSet )
     case BitmapMode_REPEAT:
         mpFS->singleElementNS( XML_a, XML_tile, FSEND );
         break;
+    case BitmapMode_STRETCH:
+        mpFS->startElementNS( XML_a, XML_stretch, FSEND );
+        mpFS->singleElementNS( XML_a, XML_fillRect, FSEND );
+        mpFS->endElementNS( XML_a, XML_stretch );
+        break;
     default:
         ;
     }