Perform rank test after solving the system.

Before this change, it was possible to adjust constraints in a way
that removes a degree of freedom and makes the sketch unsolvable,
but rank test was performed before solving the system, and an error
was not displayed immediately. Instead, a solution would seemingly
be found, but it would be very unstable--unrelated changes to
the sketch would cause rank test to fail.

To reproduce the bug, do this:
  * Draw a triangle.
  * Create a length constraint for all sides.
  * Set side lengths to a, b, and c such that a + b = c.
  * Add a line segment.

src/system.cpp

@@ -446,6 +446,11 @@ int System::Solve(Group *g, int *dof, List<hConstraint> *bad,
         return System::TOO_MANY_UNKNOWNS;
     }
 
+    // And do the leftovers as one big system
+    if(!NewtonSolve(0)) {
+        goto didnt_converge;
+    }
+
     EvalJacobian();
 
     int rank; rank = CalculateRank();
@@ -460,11 +465,6 @@ int System::Solve(Group *g, int *dof, List<hConstraint> *bad,
     // on the number of DOF.
     if(dof) *dof = mat.n - mat.m;
 
-    // And do the leftovers as one big system
-    if(!NewtonSolve(0)) {
-        goto didnt_converge;
-    }
-
     // If requested, find all the free (unbound) variables. This might be
     // more than the number of degrees of freedom. Don't always do this,
     // because the display would get annoying and it's slow.