libf3d: Improve API for [[nodiscard]] and missing this* return

[mwestphal]

• Adding [[nodiscard]] on all method where it makes sense
• Updating test to use std::ignore when needed
• Return this* whenever is was missing
• Part of Take advantage of libf3d C++17 API #1624

[github-actions]

You are modifying libf3d public API! ⚠️Please update bindings accordingly⚠️!
You can find them in their respective directories: python, java, webassembly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.64%. Comparing base (9852dab) to head (74cee5b).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1830   +/-   ##
=======================================
  Coverage   95.64%   95.64%           
=======================================
  Files         123      123           
  Lines        9763     9764    +1     
=======================================
+ Hits         9338     9339    +1     
  Misses        425      425           

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