Add support to unstroke GEOMETRYCOLLECTION

[Algunenano]

• Switched from clone to deep clone to avoid invalid access in the test. Since the test are free'ing the input geometry right after the stroke call the memory could (and did) become corrupt.
• Add support to unstroke GEOMETRYCOLLECTION. Stroking was supported but not the reverse.
• Move the unstroke tests to cu_lwstroke.c since it makes more sense to have them there.

Best reviewed per commit.

[codecov]

Codecov Report

Merging #209 into svn-trunk will increase coverage by <.01%.
The diff coverage is 93.33%.

@@              Coverage Diff              @@
##           svn-trunk     #209      +/-   ##
=============================================
+ Coverage      79.32%   79.33%   +<.01%     
=============================================
  Files            201      201              
  Lines          62973    63006      +33     
=============================================
+ Hits           49954    49986      +32     
- Misses         13019    13020       +1

Impacted Files	Coverage Δ
liblwgeom/ptarray.c	90.26% <ø> (ø)	⬆️
liblwgeom/cunit/cu_ptarray.c	99.74% <ø> (-0.05%)	⬇️
liblwgeom/cunit/cu_lwstroke.c	97.05% <100%> (+1.7%)	⬆️
liblwgeom/lwstroke.c	85.37% <60%> (+0.22%)	⬆️

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