added function for returning a block at a time

Assists with certain timeline-related functions.

app/node/output/track/track.cpp

@@ -137,6 +137,19 @@ NodeOutput* TrackOutput::track_output()
   return track_output_;
 }
 
+Block *TrackOutput::BlockContainingTime(const rational &time)
+{
+  foreach (Block* block, block_cache_) {
+    if (block->in() < time && block->out() > time) {
+      return block;
+    } else if (block->out() == time) {
+      break;
+    }
+  }
+
+  return nullptr;
+}
+
 Block *TrackOutput::NearestBlockBefore(const rational &time)
 {
   foreach (Block* block, block_cache_) {

app/node/output/track/track.h

@@ -56,6 +56,8 @@ public:
 
   NodeOutput* track_output();
 
+  Block* BlockContainingTime(const rational& time);
+
   Block* NearestBlockBefore(const rational& time);
 
   Block* NearestBlockAfter(const rational& time);