constraint solver: Fix repeat suggestions

We were not storing the previous value, causing
the first two suggestions to work, but not later
ones.

Fixes the test added in the previous commit.

gtk/gtkconstraintsolver.c

@@ -1972,6 +1972,7 @@ gtk_constraint_solver_suggest_value (GtkConstraintSolver *self,
                                      double value)
 {
   EditInfo *ei = g_hash_table_lookup (self->edit_var_map, variable);
+  double delta;
   if (ei == NULL)
     {
       g_critical ("Suggesting value '%g' but variable %p is not editable",
@@ -1987,9 +1988,10 @@ gtk_constraint_solver_suggest_value (GtkConstraintSolver *self,
       return;
     }
 
-  ei->prev_constant = value - ei->prev_constant;
+  delta = value - ei->prev_constant;
+  ei->prev_constant = value;
 
-  gtk_constraint_solver_delta_edit_constant (self, ei->prev_constant, ei->eplus, ei->eminus);
+  gtk_constraint_solver_delta_edit_constant (self, delta, ei->eplus, ei->eminus);
 }
 
 /*< private >

testsuite/gtk/constraint-solver.c

@@ -254,15 +254,27 @@ constraint_solver_edit_var_suggest (void)
   gtk_constraint_solver_suggest_value (solver, a, 2.0);
   gtk_constraint_solver_resolve (solver);
 
+  g_test_message ("Check values after first edit");
+
   g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 2.0, 0.001);
   g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (b), 2.0, 0.001);
 
   gtk_constraint_solver_suggest_value (solver, a, 10.0);
   gtk_constraint_solver_resolve (solver);
 
+  g_test_message ("Check values after second edit");
+
   g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 10.0, 0.001);
   g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (b), 10.0, 0.001);
 
+  gtk_constraint_solver_suggest_value (solver, a, 12.0);
+  gtk_constraint_solver_resolve (solver);
+
+  g_test_message ("Check values after third edit");
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 12.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (b), 12.0, 0.001);
+
   gtk_constraint_variable_unref (a);
   gtk_constraint_variable_unref (b);