Disable scatter viewer density map image broadcast when not visible

[kecnry]

This PR prevents glue from broadcasting a large binary image for scatter viewers when the density map is not in use.

This still needs testing to ensure that density maps still continue to function as designed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.07%. Comparing base (93005e7) to head (54499a8).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #467   +/-   ##
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  Coverage   86.06%   86.07%           
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  Files          90       90           
  Lines        5240     5242    +2     
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+ Hits         4510     4512    +2     
  Misses        730      730           

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[dhomeier]

I have tested this with the examples shown in #353 and found similar problems to the original issue, namely the density map not reappearing automatically when the show points switch is turned off and then back on again:

density_mark.visible is immediately set to True when show points is, but density_mark.image appears to remain set to EMPTY_IMAGE until some other update is triggering _update_rendered_image, like changing contrast, or sometimes accessing some other property of the layer (the latter not very reproducibly).
But we probably need an extra _update_rendered_image on changes to points_mode, and the same for switching from Markers.

[dhomeier]

@astrofrog the last commit resolves the initialisation in both density map and auto mode. If you have time for a quick check, I think this is ready to merge today.