[Snapshots] Refactor taking snapshots method
I refactored the snapshot routine. The new routine includes sending current drawn shapes as image data to the server. The server will use this image as overlay on the current frame. This routine works with mockup and TangoLimaVideo. Before having this routine on other video devices a cleanup and refactoring of all video_devices is needed. If done, the changes will also be made on github repository There is a complement PR for this one in mxcubecore.