Add GLUT_ALPHA for softpipe
diff --git a/progs/trivial/poly-unfilled.c b/progs/trivial/poly-unfilled.c
index e0ae463..ae092a8 100644
--- a/progs/trivial/poly-unfilled.c
+++ b/progs/trivial/poly-unfilled.c
@@ -121,7 +121,7 @@
 
     glutInitWindowPosition(0, 0); glutInitWindowSize( 250, 250);
 
-    type = GLUT_RGB;
+    type = GLUT_RGB | GLUT_ALPHA;
     type |= (doubleBuffer) ? GLUT_DOUBLE : GLUT_SINGLE;
     glutInitDisplayMode(type);