use speed value instead of media out value

This functions more or less identically to using a media out value, but the
desired speed is preserved through block length changes, even if the block's
length is reduced to zero (i.e. no rounding errors).

app/render/backend/videorenderworker.cpp

@@ -90,7 +90,7 @@ void VideoRenderWorker::HashNodeRecursively(QCryptographicHash *hash, const Node
 
         // Ignore some Block attributes when hashing
         if (input == b->media_in_input()
-            || input == b->media_out_input()
+            || input == b->speed_input()
             || input == b->length_input()) {
           continue;
         }