Add webgl support to Annulus, Wedge and AnnularWedge

[mattpap]

• AnnularWedge:
  
• Annulus:
  
• Wedge:
  

This PR:

• adds support for more GL glyphs (Annulus, Wedge, AnnularWedge)
• simplifies loading of GL glyphs in base glyphs (greatly reduced code duplication)
• greatly reduces code duplication and complexity in GL glyphs (in _set_data(), etc.)
• improves handling of settings.force_webgl in tests

TODO:

• resolve issues with GL attribute number limitations

[codecov]

Codecov Report

Merging #12704 (c8b0271) into branch-3.2 (0da7e85) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           branch-3.2   #12704   +/-   ##
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  Coverage       92.35%   92.35%           
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  Files             315      315           
  Lines           19963    19963           
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  Hits            18437    18437           
  Misses           1526     1526           

[ianthomas23]

Noting that the new WebGL AnnularWedge and Wedge glyphs do not support line joins yet. It doesn't have to be in this PR as it only really affects large line widths, but it will need to be done eventually.

[ianthomas23]

I am not really comfortable with copying someone else's code and pasting it into Bokeh. Ideally I would like the person submitting such code to demonstrate they understand it and take ownership of it in terms of supporting it in future, for two reasons:

1. I'd like to ask about the iterative Newton solver in the code block below. Why iterative? Why 5 iterations? Does it cope well with long thin ellipses? Is the 0.01 value critical or would 0.0 be OK?
2. Eventually the code for some of these new shapes is going to have to support line joins and line dashes, so it is going to need to be understood and modified.

[bryevdv]

@ianthomas23 it looks like this ellipse code has been removed for now, are you otherwise happy with this PR?

[mattpap]

I'm pretty sure I commented earlier, but maybe I didn't click "Comment". In anycase, I removed the implementation of ellipse glyph, because it's the most complex of new glyphs, one that I don't fully understand and it's not urgently needed anyway. I may get back to this at some point, when I can actually explain the details of this implementation.

[ianthomas23]

Thanks for removing the ellipse changes. As discussed, this is something we can return to later.

[mattpap]

@ianthomas23, this is already approved, but I forgot to update release notes and documentation. This PR was rebased, but only the last two commits make any changes.

[ianthomas23]

@mattpap Understood. That's OK by me, so feel free to self-merge if/when you are happy with CI.