Remove unitialized values in plplot driver

[GillesDuvert]

unitialized values were crashing GDL on Apple (OSX not permitting such things anymore).

Unitialized values are seen with Valgrind and -Wuninitialized

This patch alone does not solve all problems on Apple M1

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (a11b0a0) 41.80% compared to head (06291d7) 41.80%.

❗ Current head 06291d7 differs from pull request most recent head 7c7d38d. Consider uploading reports for the commit 7c7d38d to get more accurate results

Files	Patch %	Lines
src/plplotdriver/deprecated_wxwidgets.cpp	0.00%	2 Missing ⚠️
src/plplotdriver/deprecated_wxwidgets_app.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1674   +/-   ##
=======================================
  Coverage   41.80%   41.80%           
=======================================
  Files         356      356           
  Lines       95208    95211    +3     
  Branches    19555    19555           
=======================================
+ Hits        39803    39805    +2     
- Misses      55405    55406    +1     

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