[ENH] Faster drawing in scatterplot

[markotoplak]

Issue

Fixes #3589

Description of changes

1. Reused QPen, QBrush objects and rounded sizes to half a pixel so that the number of different symbols to draw is lower.
2. Deferred updateSpots() that gets called by pyqtgraph in set*() to paint(), where spot atlas is actually needed.

Now adult with spots of same color, shape=occupation, size=fnlwgt displays in less than 1 second instead of 10.5s as before.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3871 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3871      +/-   ##
==========================================
+ Coverage   84.97%   84.97%   +<.01%     
==========================================
  Files         378      378              
  Lines       70496    70509      +13     
==========================================
+ Hits        59905    59918      +13     
  Misses      10591    10591

[BlazZupan]

Marko, this looks great. Will this change propagate to other point-based visualizations, like in t-SNE and MDS widgets?

[markotoplak]

@BlazZupan, it will propagate to any scatterplot-based point visualization. With how large data sets did you see a difference?

[markotoplak]

@janezd, I replaced lambdas with a function.