Avoid duplicate draws in FRBMark

[astrofrog]

This does some small optimizations with FRBMark updating to avoid updating too many times when adding a layer

[codecov]

Codecov Report

Merging #249 (87ec10a) into master (b1a514a) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
- Coverage   89.17%   89.17%   -0.01%     
==========================================
  Files          82       82              
  Lines        4038     4037       -1     
==========================================
- Hits         3601     3600       -1     
  Misses        437      437              

Impacted Files	Coverage Δ
glue_jupyter/bqplot/image/frb_mark.py	63.41% <ø> (-0.88%)	⬇️
glue_jupyter/bqplot/image/layer_artist.py	88.46% <100.00%> (ø)

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[pllim]

This might have caused a problem with second image not showing up in Layers data drop-down menu as @rosteen reported in spacetelescope/jdaviz#765 (comment)

[pllim]

I folded a possible fix into #244.