pygfx: support Point symbols

[gselzer]

Somehow point symbols were overlooked to this point for pygfx. This PR adds them in.

@tlambert03, since you wrote scenex.model.SymbolName: there are many symbol types that pertain only to pygfx that aren't in that list, and conversely some symbols that the pygfx backend can't (without a custom marker type, which is some additional effort) handle. We should probably come up with a strategy to address that for each backend (maybe just make SymbolName the union of all supported markers, and only support some in each backend?) - but we can address this in another PR, for sure

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.41%. Comparing base (6e10ae7) to head (5bc6418).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   88.38%   88.41%   +0.03%     
==========================================
  Files          62       62              
  Lines        3064     3074      +10     
==========================================
+ Hits         2708     2718      +10     
  Misses        356      356              

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[Copilot]

Pull request overview

Adds point symbol support to the pygfx backend by translating scenex.model.SymbolName values into pygfx.PointsMarkerMaterial.marker values, with warnings for unsupported symbols.

Changes:

• Implement _snx_set_symbol for the pygfx Points adaptor, including a few alias translations and a warning path for unsupported symbols.
• Add a new parametrized test covering all SymbolName values, including alias handling and expected warnings for unsupported markers.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/scenex/adaptors/_pygfx/_points.py	Implements pygfx marker selection logic for Points.symbol, with aliases and warnings for unsupported symbols.
tests/adaptors/_pygfx/test_points.py	Adds coverage for symbol updates across all SymbolName options, including unsupported-symbol warning behavior.

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