Flip extrusion normal when switching between union and difference.

This is done because a meaningful union extrusion is almost never
a meaningful difference extrusion, and saves a bunch of common
manual work.

To avoid creating invalid sketches this isn't done when there are any
constraints.

src/group.cpp

@@ -51,6 +51,26 @@ bool Group::IsVisible(void) {
     return true;
 }
 
+int Group::GetNumConstraints(void) {
+    int num = 0;
+    for(int i = 0; i < SK.constraint.n; i++) {
+        Constraint *c = &SK.constraint.elem[i];
+        if(c->group.v != h.v) continue;
+        num++;
+    }
+    return num;
+}
+
+Vector Group::ExtrusionGetVector() {
+    return Vector::From(h.param(0), h.param(1), h.param(2));
+}
+
+void Group::ExtrusionForceVectorTo(const Vector &v) {
+    SK.GetParam(h.param(0))->val = v.x;
+    SK.GetParam(h.param(1))->val = v.y;
+    SK.GetParam(h.param(2))->val = v.z;
+}
+
 void Group::MenuGroup(int id) {
     Group g = {};
     g.visible = true;

src/sketch.h

@@ -236,6 +236,9 @@ public:
     void AddEq(IdList<Equation,hEquation> *l, Expr *expr, int index);
     void GenerateEquations(IdList<Equation,hEquation> *l);
     bool IsVisible(void);
+    int GetNumConstraints();
+    Vector ExtrusionGetVector();
+    void ExtrusionForceVectorTo(const Vector &v);
 
     // Assembling the curves into loops, and into a piecewise linear polygon
     // at the same time.

src/textscreens.cpp

@@ -191,7 +191,15 @@ void TextWindow::ScreenChangeGroupOption(int link, uint32_t v) {
         case 'k': g->skipFirst = true; break;
         case 'K': g->skipFirst = false; break;
 
-        case 'c': g->meshCombine = v; break;
+        case 'c':
+            // When an extrude group is first created, it's positioned for a union
+            // extrusion. If no constraints were added, flip it when we switch between
+            // union and difference modes to avoid manual work doing the smae.
+            if(g->meshCombine != v && g->GetNumConstraints() == 0) {
+                g->ExtrusionForceVectorTo(g->ExtrusionGetVector().Negated());
+            }
+            g->meshCombine = v;
+            break;
 
         case 'P': g->suppress = !(g->suppress); break;