player: API to configure subtitle background color
@philn
Submitted by Philippe Normand Link to original bug (#795851)
Description
This would be useful when using external subtitles for a video which already has burned-in subtitles. Setting the background color to opaque black would make the overlayed external subtitles readable over the burned-in subtitles.
A subtitle-background-color property would then be needed in playbin(3), uridecodebin, subtitleoverlay and ...?
For textoverlay, see https://bugzilla.gnome.org/show_bug.cgi?id=795850