Reorganise antialiasing code

[ianthomas23]

This is a reorganisation of some of the antialiasing code to make it easier to understand. It uses a NamedTuple for AntialiasingStage2 rather than just a tuple, and moves the antialiasing code from various locations to a new antialias.py source code file.

This is a precursor to the AntialiasingStage2 becoming more complicated.

[codecov]

Codecov Report

Merging #1245 (65b1f39) into main (d534e2d) will decrease coverage by 0.01%.
The diff coverage is 92.62%.

@@            Coverage Diff             @@
##             main    #1245      +/-   ##
==========================================
- Coverage   83.42%   83.42%   -0.01%     
==========================================
  Files          35       35              
  Lines        8858     8869      +11     
==========================================
+ Hits         7390     7399       +9     
- Misses       1468     1470       +2     

Impacted Files	Coverage Δ
datashader/glyphs/__init__.py	100.00% <ø> (ø)
datashader/compiler.py	88.32% <75.00%> (-0.34%)	⬇️
datashader/reductions.py	77.87% <81.25%> (ø)
datashader/antialias.py	96.55% <96.55%> (ø)
datashader/glyphs/line.py	92.86% <100.00%> (-0.19%)	⬇️

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