preserve container depth across yyjson_incr_read calls

[two-heart]

Fixes #298

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.46%. Comparing base (db37a64) to head (30fd6c0).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/yyjson.c	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
- Coverage   98.47%   98.46%   -0.02%     
==========================================
  Files           2        2              
  Lines        7743     7747       +4     
==========================================
+ Hits         7625     7628       +3     
- Misses        118      119       +1     

Flag	Coverage Δ
unittests	98.46% <75.00%> (-0.02%)	⬇️

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[ibireme]

Thanks!