Create a quick view of the frames being taken

To aide in commissioning (especially to make sure that we don't overload the detector) it would be good to be able to view a video stream of frames coming out of the Eiger.

Obviously, the eiger can be triggered at kHz and we can't show that speed so downsampling to ~10Hz is fine

## Acceptance Criteria
- There is a video stream on the synoptic for data coming out of the eiger
- This shows the data at the same time as it is being written to disk
- The stream updates at ~10Hz
- The min/max values displayed on the frames is configurable e.g. to work around dead pixels and gaps

Start by locating the Eiger frame acquisition and data-writing entry points, then find how the synoptic currently receives or displays detector data. Trace whether a stream can remain synchronized with disk writes and where frame downsampling and configurable min/max display values belong. Done means the synoptic shows the written data at about 10 Hz without overloading the detector.

Written by the indexing model from the issue text.