[ENH] Paint Data: Slight optimization

[ales-erjavec]

Issue

Ref: #6971

Description of changes

Slight optimization. Most of the time still spent in ScatterPlotItem constructor.

Includes

• Code changes
• Tests
• Documentation

[borondics]

@ales-erjavec, this seems to fix things. I don't get a slowdown anymore. I tried Paint Data alone and also when connected to Distances and Hierarchical Clustering with auto commit on up to about 1000 points. The widget is now responsive.

Previously, when there were many (~500 points) the painting didn't catch up with the mouse action, meaning that when I released the button, the points still kept coming in.

Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 72.41379% with 8 lines in your changes missing coverage. Please review.

Project coverage is 88.46%. Comparing base (876acb2) to head (ca65126).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6990      +/-   ##
==========================================
- Coverage   88.47%   88.46%   -0.01%     
==========================================
  Files         332      332              
  Lines       73294    73314      +20     
==========================================
+ Hits        64847    64859      +12     
- Misses       8447     8455       +8