textoverlay silent property looks not well implemented

[bcxa sz]

Hi Developers,

The 'silent' property (to hide and display subtitle during playback)
of official gstreamer textoverlay is also not well implemented.

In textoverlay gst_text_overlay_video_chain(), it just push pure video
data but without consuming the text data. Then
gst_text_overlay_text_chain() will blocked to waiting for the text
data consumed.

For subtitle mixed with av in the same file, the demux will block at
the text demuxing since text data not consumed then will not demux any
followed audio and video data.

best regards

BCXA