made vast improvements to rendering engine and node structure

This was many changes that were largely fundamentally related. They included:
- More const modifiers to enforce read only node graphs
- Support for fragment and vertex shaders from the nodes
- Support for node code loaded externally (embedded into the binary)
- Fixed issue preventing two textures from being used in a shader
- Removed several unused functions and cleaned up code
- Fixed video media node misreading its matrix input

app/project/item/sequence/sequence.cpp

@@ -70,6 +70,7 @@ void Sequence::add_default_nodes()
   Node* audio_track_output = timeline_output_->track_list(TrackType::kTrackTypeAudio)->AddTrack();
   NodeParam::ConnectEdge(video_track_output->output(), viewer_output_->texture_input());
   NodeParam::ConnectEdge(audio_track_output->output(), viewer_output_->samples_input());
+  //timeline_output_->track_list(TrackType::kTrackTypeVideo)->AddTrack();
 
   // Update the timebase on these nodes
   set_video_params(video_params_);