Fix #131. Prevent UI freeze by having a timeout when finding which constraints can be removed to fix jacobian.

src/sketch.h

@@ -189,6 +189,7 @@ public:
     struct {
         SolveResult         how;
         int                 dof;
+        bool                timeout;
         List<hConstraint>   remove;
     } solved;
 

src/system.cpp

@@ -355,10 +355,17 @@ void System::WriteEquationsExceptFor(hConstraint hc, Group *g) {
 }
 
 void System::FindWhichToRemoveToFixJacobian(Group *g, List<hConstraint> *bad, bool forceDofCheck) {
+    auto time = GetMilliseconds();
+    g->solved.timeout = false;
     int a;
 
     for(a = 0; a < 2; a++) {
         for(auto &con : SK.constraint) {
+            if((GetMilliseconds() - time) > 1500) { // todo: make timeout configurable
+                g->solved.timeout = true;
+                return;
+            }
+
             ConstraintBase *c = &con;
             if(c->group != g->h) continue;
             if((c->type == Constraint::Type::POINTS_COINCIDENT && a == 0) ||

src/textscreens.cpp

@@ -578,6 +578,11 @@ void TextWindow::ShowGroupSolveInfo() {
             c->DescriptionString().c_str());
     }
 
+    if(g->solved.timeout) {
+        Printf(true,  "%FxSome items in list have been ommitted%Fd");
+        Printf(false,  "%Fxbecause the operation timed out.%Fd");
+    }
+
     Printf(true,  "It may be possible to fix the problem ");
     Printf(false, "by selecting Edit -> Undo.");