Add manifold_meshgl_merge to c bindings

[harmanpa]

Add manifold_meshgl_merge to call MeshGL::Merge() on a duplicate mesh via the c bindings

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 89.22%. Comparing base (d437097) to head (d6f68bf).
Report is 26 commits behind head on master.

Files	Patch %	Lines
bindings/c/manifoldc.cpp	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
- Coverage   91.84%   89.22%   -2.62%     
==========================================
  Files          37       61      +24     
  Lines        4976     8444    +3468     
  Branches        0      928     +928     
==========================================
+ Hits         4570     7534    +2964     
- Misses        406      910     +504     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[pca006132]

Why do we need to duplicate the mesh here?

[harmanpa]

Only because from_c returns a const so the compiler wouldn't allow it otherwise

[pca006132]

I see. LGTM.