Add support for glyph decorations/markers

[mattpap]

Early work in progress. Glyph decorations were added previously, but only in a very limited and experimental fashion. This PR's goal is to improve upon that and establish decorations as a feature and add it to all relevant glyphs. The specific applications of this feature are directed graphs and vector fields.

Example showing Arc, Ray, MultiLine and Bezier glyphs:

fixes #8129

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6560e98) 92.54% compared to head (aa9c5d4) 92.54%. Report is 1 commits behind head on branch-3.4.

Additional details and impacted files

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##           branch-3.4   #13619   +/-   ##
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  Coverage       92.54%   92.54%           
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  Files             323      323           
  Lines           20493    20493           
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  Hits            18965    18965           
  Misses           1528     1528           

[bryevdv]

@mattpap can you briefly expand on what this will look like from a user perspective? This came up at Scipy in regards to a specialized kind of meteorological plot, and it seems that an ability to add custom arrowheads would be a solution, once there are "vectorized arrows" as a basis, which this PR seems to provide.