Refactor screenshot tester to use offscreen rendering utils.

[Kietyo]

No description provided.

[codecov-commenter]

Codecov Report

Patch coverage: 35.29% and project coverage change: -0.19% ⚠️

Comparison is base (37e5917) 51.65% compared to head (7160bdc) 51.47%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1819      +/-   ##
==========================================
- Coverage   51.65%   51.47%   -0.19%     
==========================================
  Files        1750     1750              
  Lines      102821   102826       +5     
  Branches    14521    14519       -2     
==========================================
- Hits        53114    52931     -183     
- Misses      45666    45912     +246     
+ Partials     4041     3983      -58     

Flag	Coverage Δ
unittests	51.47% <35.29%> (-0.19%)	⬇️

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Files Changed	Coverage Δ
...kotlin/korlibs/korge/testing/KorgeOffscreenTest.kt	70.49% <0.00%> (+49.18%)	⬆️
...Main/kotlin/korlibs/korge/testing/testing_utils.kt	17.47% <14.28%> (-2.14%)	⬇️
...ain/kotlin/korlibs/korge/testing/BitmapAsserter.kt	9.16% <66.66%> (+9.16%)	⬆️
...lin/korlibs/korge/testing/KorgeScreenshotTester.kt	78.26% <100.00%> (+2.17%)	⬆️

... and 53 files with indirect coverage changes

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