Add GLUT_ALPHA for softpipe
diff --git a/progs/trivial/tri-flat.c b/progs/trivial/tri-flat.c
index fb2cdbd..0583a2a 100644
--- a/progs/trivial/tri-flat.c
+++ b/progs/trivial/tri-flat.c
@@ -119,7 +119,7 @@
 
     glutInitWindowPosition(0, 0); glutInitWindowSize( 250, 250);
 
-    type = GLUT_RGB;
+    type = GLUT_RGB | GLUT_ALPHA;
     type |= (doubleBuffer) ? GLUT_DOUBLE : GLUT_SINGLE;
     glutInitDisplayMode(type);