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/block/clip/clip.cpp

@@ -27,7 +27,7 @@ ClipBlock::ClipBlock()
   AddParameter(texture_input_);
 }
 
-Node *ClipBlock::copy()
+Node *ClipBlock::copy() const
 {
   ClipBlock* c = new ClipBlock();
 
@@ -36,27 +36,27 @@ Node *ClipBlock::copy()
   return c;
 }
 
-Block::Type ClipBlock::type()
+Block::Type ClipBlock::type() const
 {
   return kClip;
 }
 
-QString ClipBlock::Name()
+QString ClipBlock::Name() const
 {
   return tr("Clip");
 }
 
-QString ClipBlock::id()
+QString ClipBlock::id() const
 {
   return "org.olivevideoeditor.Olive.clip";
 }
 
-QString ClipBlock::Description()
+QString ClipBlock::Description() const
 {
   return tr("A time-based node that represents a media source.");
 }
 
-NodeInput *ClipBlock::texture_input()
+NodeInput *ClipBlock::texture_input() const
 {
   return texture_input_;
 }
@@ -79,7 +79,7 @@ void ClipBlock::InvalidateCache(const rational &start_range, const rational &end
   }
 }
 
-TimeRange ClipBlock::InputTimeAdjustment(NodeInput *input, const TimeRange &input_time)
+TimeRange ClipBlock::InputTimeAdjustment(NodeInput *input, const TimeRange &input_time) const
 {
   if (input == texture_input_) {
     return TimeRange(SequenceToMediaTime(input_time.in()), SequenceToMediaTime(input_time.out()));
@@ -88,11 +88,8 @@ TimeRange ClipBlock::InputTimeAdjustment(NodeInput *input, const TimeRange &inpu
   return Block::InputTimeAdjustment(input, input_time);
 }
 
-QVariant ClipBlock::Value(NodeOutput *output)
+NodeValueTable ClipBlock::Value(const NodeValueDatabase &value) const
 {
-  if (output == buffer_output()) {
-    // We just pass through the texture here, the renderer should have gotten the correct time from InputTimeAdjustment
-    return texture_input()->value();
-  }
-  return Block::Value(output);
+  // We just pass through the data here, the renderer should have gotten the correct time from InputTimeAdjustment
+  return value.Merge();
 }