Add GLUT_ALPHA for softpipe
diff --git a/progs/trivial/quad-flat.c b/progs/trivial/quad-flat.c
index d6e9f02..bd8be7c 100644
--- a/progs/trivial/quad-flat.c
+++ b/progs/trivial/quad-flat.c
@@ -130,7 +130,7 @@
 
     glutInitWindowPosition(0, 0); glutInitWindowSize( 250, 250);
 
-    type = GLUT_RGB;
+    type = GLUT_RGB | GLUT_ALPHA;
     type |= (doubleBuffer) ? GLUT_DOUBLE : GLUT_SINGLE;
     glutInitDisplayMode(type);