sweeping node graph changes for new rendering pipeline

The new rendering pipeline strives to simplify the nodes themselves as much
as possible and move much of the logic to an external rendering engine. This
change removes all of the responsibilities that no longer belong to the
nodes themselves and will soon be folded into the renderer.

app/node/block/block.cpp

@@ -64,11 +64,11 @@ void Block::set_length(const rational &length)
 {
   Q_ASSERT(length > 0);
 
-  Lock();
+  LockUserInput();
 
   length_ = length;
 
-  Unlock();
+  UnlockUserInput();
 
   Refresh();
 }
@@ -84,7 +84,7 @@ void Block::set_length_and_media_in(const rational &length)
 
 Block *Block::previous()
 {
-  return ValueToPtr<Block>(previous_input_->get_value(0));
+  return ValueToPtr<Block>(previous_input_->get_realtime_value_of_connected_output());
 }
 
 Block *Block::next()
@@ -97,11 +97,8 @@ NodeInput *Block::previous_input()
   return previous_input_;
 }
 
-QVariant Block::Value(NodeOutput *output, const rational &in, const rational &out)
+QVariant Block::Value(NodeOutput *output)
 {
-  Q_UNUSED(in)
-  Q_UNUSED(out)
-
   if (output == block_output_) {
     // Simply set the output value to a pointer to this Block
     return PtrToValue(this);
@@ -179,7 +176,7 @@ const rational &Block::media_in()
 
 void Block::set_media_in(const rational &media_in)
 {
-  Lock();
+  LockUserInput();
 
   if (media_in_ != media_in) {
     media_in_ = media_in;
@@ -188,7 +185,7 @@ void Block::set_media_in(const rational &media_in)
     SendInvalidateCache(in(), out());
   }
 
-  Unlock();
+  UnlockUserInput();
 }
 
 const QString &Block::block_name()