Xext: Fix edge case with {Positive, Negative}Transition triggers.

The {Positive,Negative}Transition triggers only fire when the counter
goes from strictly {below,above} the threshold.  If
SyncComputeBracketValues gets called exactly at this threshold we may update
the bracket values so that the counter is not updated past the threshold.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit b55bf24)

Xext/sync.c

@@ -959,6 +959,17 @@ SyncComputeBracketValues(SyncCounter *pCounter)
 	    {
 		psci->bracket_less = pTrigger->test_value;
 		pnewltval = &psci->bracket_less;
+	    } else if (XSyncValueEqual(pCounter->value, pTrigger->test_value) &&
+		       XSyncValueLessThan(pTrigger->test_value,
+					  psci->bracket_greater))
+	    {
+	        /*
+		 * The value is exactly equal to our threshold.  We want one
+		 * more event in the positive direction to ensure we pick up
+		 * when the value *exceeds* this threshold.
+		 */
+	        psci->bracket_greater = pTrigger->test_value;
+		pnewgtval = &psci->bracket_greater;
 	    }
 	}
         else if (pTrigger->test_type == XSyncPositiveTransition &&
@@ -969,6 +980,17 @@ SyncComputeBracketValues(SyncCounter *pCounter)
 	    {
 		psci->bracket_greater = pTrigger->test_value;
 		pnewgtval = &psci->bracket_greater;
+	    } else if (XSyncValueEqual(pCounter->value, pTrigger->test_value) &&
+		       XSyncValueGreaterThan(pTrigger->test_value,
+					     psci->bracket_less))
+	    {
+	        /*
+		 * The value is exactly equal to our threshold.  We want one
+		 * more event in the negative direction to ensure we pick up
+		 * when the value is less than this threshold.
+		 */
+	        psci->bracket_less = pTrigger->test_value;
+		pnewltval = &psci->bracket_less;
 	    }
 	}
     } /* end for each trigger */