overhauled render classes to support video AND audio

Major refactoring work to try sharing as much code as possible between the
video renderers and audio renderers, as well as make them as
platform-independent as possible.

app/render/backend/videorenderframecache.cpp

@@ -67,6 +67,19 @@ void VideoRenderFrameCache::RemoveHash(const rational &time)
   time_hash_map_.remove(time);
 }
 
+void VideoRenderFrameCache::Truncate(const rational &time)
+{
+  QMap<rational, QByteArray>::iterator i = time_hash_map_.begin();
+
+  while (i != time_hash_map_.end()) {
+    if (i.key() >= time) {
+      i = time_hash_map_.erase(i);
+    } else {
+      i++;
+    }
+  }
+}
+
 void VideoRenderFrameCache::RemoveHashFromCurrentlyCaching(const QByteArray &hash)
 {
   currently_caching_lock_.lock();