make nodes more const friendly and prepare their functions for stateless work

Since the nodes won't be holding any rendering data themselves in this system,
we may as well enforce some level of non-write access by setting all the
functions to const. They were already const-friendly, they just weren't
labelled as such.

app/node/blend/blend.cpp

@@ -34,22 +34,22 @@ BlendNode::BlendNode()
   AddParameter(texture_output_);
 }
 
-QString BlendNode::Category()
+QString BlendNode::Category() const
 {
   return tr("Blend");
 }
 
-NodeInput *BlendNode::base_input()
+NodeInput *BlendNode::base_input() const
 {
   return base_input_;
 }
 
-NodeInput *BlendNode::blend_input()
+NodeInput *BlendNode::blend_input() const
 {
   return blend_input_;
 }
 
-NodeOutput *BlendNode::texture_output()
+NodeOutput *BlendNode::texture_output() const
 {
   return texture_output_;
 }