Constraints can override their own number of solver iterations (either more or less than the default) or leave it default (-1)

Bump version to 2.80

src/BulletDynamics/ConstraintSolver/btSolverConstraint.h

@@ -51,6 +51,8 @@ ATTRIBUTE_ALIGNED64 (struct)	btSolverConstraint
 		btScalar	m_unusedPadding0;
 	};
 
+	int	m_overrideNumSolverIterations;
+
 	union
 	{
 		int			m_frictionIndex;