Add a workaround for OpenGL, which likes to tesselate with

zero-area triangles, apparently. And make the number of line
segments used to approximate a triangle depend on its scale on
screen.

[git-p4: depot-paths = "//depot/solvespace/": change = 1738]

entity.cpp

@@ -746,6 +746,7 @@ void Entity::DrawOrGetDistance(int order) {
             break;
         }
 
+#define CIRCLE_SIDES(r) (7 + (int)(10*((sqrt(r))*SS.GW.scale)/20))
         case ARC_OF_CIRCLE: {
             if(order >= 0 && order != 1) break;
             Vector c  = SS.GetEntity(point[0])->PointGetNum();
@@ -760,7 +761,7 @@ void Entity::DrawOrGetDistance(int order) {
             double thetaa, thetab, dtheta;
             ArcGetAngles(&thetaa, &thetab, &dtheta);
 
-            int i, n = (int)((20*dtheta)/(2*PI));
+            int i, n = 3 + (int)(CIRCLE_SIDES(ra)*dtheta/(2*PI));
             Vector prev = pa;
             for(i = 1; i <= n; i++) {
                 double theta = thetaa + (dtheta*i)/n;
@@ -781,7 +782,7 @@ void Entity::DrawOrGetDistance(int order) {
             Vector center = SS.GetEntity(point[0])->PointGetNum();
             Vector u = q.RotationU(), v = q.RotationV();
 
-            int i, c = 20;
+            int i, c = CIRCLE_SIDES(r); 
             Vector prev = u.ScaledBy(r).Plus(center);
             for(i = 1; i <= c; i++) {
                 double phi = (2*PI*i)/c;

graphicswin.cpp

@@ -1203,7 +1203,7 @@ void GraphicsWindow::Paint(int w, int h) {
         glDisable(GL_LIGHTING);
         glxLockColorTo(0, 1, 0);
         glEnable(GL_DEPTH_TEST);
-//        glxDebugMesh(&br); 
+        glxDebugMesh(&br); 
 
         br.Clear();
         FreeAllTemporary();

mesh.cpp

@@ -7,6 +7,10 @@ void SMesh::Clear(void) {
 void SMesh::AddTriangle(Vector n, Vector a, Vector b, Vector c) {
     Vector ab = b.Minus(a), bc = c.Minus(b);
     Vector np = ab.Cross(bc);
+    if(np.Magnitude() < 1e-10) {
+        // ugh; gl sometimes tesselates to collinear triangles
+        return;
+    }
     if(np.Dot(n) > 0) {
         AddTriangle(a, b, c);
     } else {