gstbasetextoverlay: emit signal when text chain is waiting for video buffer
Submitted by Andreas Frisch
in a scenario where subtitles aren't rendered with video fps but with actually just one video buffer per subtitle buffer, it's necessary to push a single video buffer into the subtitle renderer as soon as it has a text buffer queued.
implementing this "wait-video" signal was the only way i could figure out to achieve the correct timing for this.
note that pushing the video frame(s) has to be done with a seperate thread, e.g. with an appsrc, pushing it directly to the pad will result in a deadlock because the signal is emitted within a locked section of the text chain func
suggested patch to be attached