feat: Contrasting clim handles

[gselzer]

When I was filing #152, I noticed that the particular dataset/view parameters make the clim handles nearly invisible. Only powerusers will be able to find them:

I don't have any great ideas, but ideally the clims/gamma handle would strongly contrast with the histogram mesh. We may want something similar for any annotations added to the canvas (#46, #47)

Probably a low-priority issue though, because the conditions needed to create this situation are likely rare in practice.