Adding a dynamic scale bar

[MrMupfel]

I would be nice to have a dynamic scale bar that updates while zooming in.

Thank you very much for making this open source! I aim to embed this into our labs server to display very large images from electron microscopy, so a scale bar would be a huge benefit.

Cheers,
Flo

[MrMupfel]

I am currently working on this here -> https://github.com/MrMupfel/vizarr (I also added the option to draw ROIs (which will eventually be saved by a Django backend)). This is by far not ready for a pull request but its working for me so far :)