Add GLUT_ALPHA for softpipe
diff --git a/progs/trivial/quad-offset-factor.c b/progs/trivial/quad-offset-factor.c
index 861d346..becc885 100644
--- a/progs/trivial/quad-offset-factor.c
+++ b/progs/trivial/quad-offset-factor.c
@@ -149,7 +149,7 @@
 
     glutInitWindowPosition(0, 0); glutInitWindowSize( 250, 250);
 
-    type = GLUT_RGB | GLUT_DEPTH;
+    type = GLUT_RGB | GLUT_ALPHA | GLUT_DEPTH;
     type |= (doubleBuffer) ? GLUT_DOUBLE : GLUT_SINGLE;
     glutInitDisplayMode(type);