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/input/media/video/video.cpp

@@ -18,27 +18,27 @@ VideoInput::VideoInput()
   AddParameter(texture_output_);
 }
 
-Node *VideoInput::copy()
+Node *VideoInput::copy() const
 {
   return new VideoInput();
 }
 
-QString VideoInput::Name()
+QString VideoInput::Name() const
 {
   return tr("Video Input");
 }
 
-QString VideoInput::id()
+QString VideoInput::id() const
 {
   return "org.olivevideoeditor.Olive.videoinput";
 }
 
-QString VideoInput::Category()
+QString VideoInput::Category() const
 {
   return tr("Input");
 }
 
-QString VideoInput::Description()
+QString VideoInput::Description() const
 {
   return tr("Import a video footage stream.");
 }
@@ -48,17 +48,17 @@ void VideoInput::Release()
   MediaInput::Release();
 }
 
-NodeInput *VideoInput::matrix_input()
+NodeInput *VideoInput::matrix_input() const
 {
   return matrix_input_;
 }
 
-NodeOutput *VideoInput::texture_output()
+NodeOutput *VideoInput::texture_output() const
 {
   return texture_output_;
 }
 
-QString VideoInput::Code(NodeOutput *output)
+QString VideoInput::Code(NodeOutput *output) const
 {
   if (output == texture_output()) {
     return "#version 110\n"