rostextsrc buffer duration needs to be set by message contents

[BrettRD]

The gstreamer textoverlay plugin requires buffer duration flags be set to strictly correct values and freezes the video if buffer duration is set to 0, GST_CLOCK_TIME_NONE, or if the duration expires.

To support textoverlay, rostextsrc should allow the user to configure the buffer timestamps and duration on a per-message basis.

[BrettRD]

A temporary workaround for textoverlay uses textrender and compositor

gst-launch-1.0 --gst-plugin-path=install/gst_bridge/lib/gst_bridge/ videotestsrc ! video/x-raw,width=640,height=480 ! mix. rostextsrc ! textrender ! video/x-raw,width=640,height=480 ! compositor name=mix ! videoconvert ! autovideosink

compositor has a z-order property on each pad, this pipeline exploits the default order to place the text in front. The image sources do not need to be the same size or shape; the position and z-order can be set explicitly for multiple sources, see compositor for details
textrender seems to ignore empty strings, but will happily produce a frame containing only whitespace.

[BrettRD]

Adding 'good first issue'
If you have an idea and a use-case for a ros message carrying subtitle timing info or similar, I want to hear it.