mesa: Only complain about an infinite loop in a swrast program once.

Chances are, if one fragment looped badly, others will too, and
debugging output gets overwhelmed by the looping complaints.
diff --git a/src/mesa/shader/prog_execute.c b/src/mesa/shader/prog_execute.c
index f85c651..b6da344 100644
--- a/src/mesa/shader/prog_execute.c
+++ b/src/mesa/shader/prog_execute.c
@@ -1788,7 +1788,11 @@
 
       numExec++;
       if (numExec > maxExec) {
-         _mesa_problem(ctx, "Infinite loop detected in fragment program");
+	 static GLboolean reported = GL_FALSE;
+	 if (!reported) {
+	    _mesa_problem(ctx, "Infinite loop detected in fragment program");
+	    reported = GL_TRUE;
+	 }
          return GL_TRUE;
       }