Support antialiased line self_intersect in categorical aggs

[ianthomas23]

PR #1081 (antialiased lines for categorical aggs) didn't include support for 1-stage agg (self_intersect=True) as introduced by PR #1072. This PR fixes it.

Demo of fix:

Note the self-intersections near the center of the second plot. Before this fix the second plot looked the same as the first.

[codecov]

Codecov Report

Merging #1083 (7407b39) into master (1cacb25) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1083   +/-   ##
=======================================
  Coverage   83.35%   83.35%           
=======================================
  Files          34       34           
  Lines        7490     7491    +1     
=======================================
+ Hits         6243     6244    +1     
  Misses       1247     1247           

Impacted Files	Coverage Δ
datashader/core.py	88.11% <100.00%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1cacb25...7407b39. Read the comment docs.