lokdialog: debug red-colored boundary for last painted region

Easier to see what's going on inside the LOK dialog.

Change-Id: I59974200af3d2356b7c27eb3dd4032a59c00dbed
Reviewed-on: https://gerrit.libreoffice.org/51263Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: pranavk <pranavk@collabora.co.uk>

libreofficekit/qa/gtktiledviewer/gtv-lok-dialog.cxx

@@ -117,7 +117,13 @@ gtv_lok_dialog_draw(GtkWidget* pDialogDrawingArea, cairo_t* pCairo, gpointer)
     cairo_surface_mark_dirty(pSurface);
 
     cairo_set_source_surface(pCairo, pSurface, aRect.x, aRect.y);
+    // paint the dialog image
     cairo_paint(pCairo);
+
+    // debug red-colored border around the painted region
+    cairo_set_source_rgb(pCairo, 1.0, 0, 0);
+    cairo_rectangle(pCairo, aRect.x, aRect.y, nWidth, nHeight);
+    cairo_stroke(pCairo);
 }
 
 static gboolean