Boardview: Extract Shadowmap

[SJuliez]

This moves the shadowmap creation out of BoardView. The goal is to make BoardView smaller so it becomes more palatable and I can extract the parts from it that deal with Entity, i.e. Total Warfare stuff and get down to a base boardview that I can use for other game types.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 212 lines in your changes are missing coverage. Please review.

Project coverage is 29.11%. Comparing base (0742934) to head (00711c0).
Report is 73 commits behind head on master.

❗ Current head 00711c0 differs from pull request most recent head 63f33ae. Consider uploading reports for the commit 63f33ae to get more accurate results

Files	Patch %	Lines
...client/ui/swing/boardview/TerrainShadowHelper.java	0.00%	196 Missing ⚠️
megamek/src/megamek/common/util/ImageUtil.java	0.00%	11 Missing ⚠️
...c/megamek/client/ui/swing/boardview/BoardView.java	0.00%	4 Missing ⚠️
...c/megamek/client/ui/swing/tileset/EntityImage.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5318      +/-   ##
============================================
+ Coverage     29.09%   29.11%   +0.02%     
- Complexity    13459    13473      +14     
============================================
  Files          2390     2390              
  Lines        260774   260685      -89     
  Branches      46766    46743      -23     
============================================
+ Hits          75868    75899      +31     
+ Misses       181232   181121     -111     
+ Partials       3674     3665       -9     

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