nodes: improved changed signal processing

Improves stability and cache reliability.

Earlier iterations were prone to skipping necessary signals (usually
leading to some sort of assert fail), particularly when track
optimizations were used. Those optimizations have been moved to the
viewer node so there's a higher degree of control over which signals
get optimized and in which ways.

app/node/block/block.cpp

@@ -355,16 +355,6 @@ NodeInput *Block::speed_input() const
   return speed_input_;
 }
 
-void Block::InvalidateCache(const TimeRange &range, NodeInput *from, NodeInput *source)
-{
-  if (range.out() <= in() || range.in() >= out()) {
-    // Ignore this range
-    return;
-  }
-
-  Node::InvalidateCache(TimeRange(qMax(range.in(), in()), qMin(range.out(), out())), from, source);
-}
-
 void Block::Hash(QCryptographicHash &, const rational &) const
 {
   // A block does nothing by default, so we hash nothing