reduce UI image latency

Current ui implementation induces about 1 second latency in the image visu.

This might be due to OSD drawing, but maybe also due to inherent GTK latency.

If i check the image in the rqt image visu the latency is way smaller. how could we reduce the latency?

Ideas how to optimize:

• Draw OSD in separate thread as overlay and put this on top of fast image visu (OSD latency might be still high but ignore this)
• Speedup current implementation somehow