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/node/graph.cpp

@@ -108,12 +108,3 @@ bool NodeGraph::ContainsNode(Node *n)
 {
   return (n->parent() == this);
 }
-
-void NodeGraph::Release()
-{
-  QList<Node*> all_nodes = nodes();
-
-  foreach (Node* n, all_nodes) {
-    n->Release();
-  }
-}