various stuff in opengl demos such as a better model,
stable trackball for the fly navigation mode, and started
to put some GUI elements...

demos/opengl/camera.cpp

diff --git a/demos/opengl/camera.cpp b/demos/opengl/camera.cpp
index 26118e6..f704771 100644
--- a/demos/opengl/camera.cpp
+++ b/demos/opengl/camera.cpp
@@ -42,8 +42,7 @@
     mVpX = 0;
     mVpY = 0;
 
-    setPosition(Vector3f::Constant(50.));
-
+    setPosition(Vector3f::Constant(100.));
     setTarget(Vector3f::Zero());
 }
 
@@ -179,6 +178,14 @@
     mViewIsUptodate = true;
 }
 
+void Camera::localRotate(const Quaternionf& q)
+{
+    float dist = (position() - mTarget).norm();
+    setOrientation(orientation() * q);
+    mTarget = position() + dist * direction();
+    mViewIsUptodate = false;
+}
+
 void Camera::zoom(float d)
 {
     float dist = (position() - mTarget).norm();