downloadbuffer pushing first data buffer before stream start event

downloadbuffer is pushing the first buffer before pushing Stream start and Segment event to the next element. This happens when the source pad of download buffer works in push mode. the download_buffer_loop() function will be waiting for data to available in the temporary file, once it is available it will push the buffer, before even pushing stream start and segment event to next element.

This might cause the downstream element to ignore that first buffer.