Viewport size initialization.

Former-commit-id: d8d9e0a605618b56499a83ee801dc072a7cabe09

include/igl/viewer/ViewerCore.cpp

@@ -433,6 +433,8 @@ IGL_INLINE igl::viewer::ViewerCore::ViewerCore()
   line_width = 0.5f;
   is_animating = false;
   animation_max_fps = 30.;
+
+  viewport.setZero();
 }
 
 IGL_INLINE void igl::viewer::ViewerCore::init()