Cleanup & fix: Removes premultiplied complexity in shaders

[soywiz]

Since now shaders are expected to have premultiplied alpha colors as input and output, we can simplify & remove some old code and prevent some issues.

Based on this #1159 from @BioRyajenka with further cleanups + updating tests

[codecov-commenter]

Codecov Report

Base: 54.25% // Head: 54.23% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (f73df30) compared to base (8dac89f).
Patch coverage: 86.95% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1162      +/-   ##
==========================================
- Coverage   54.25%   54.23%   -0.03%     
==========================================
  Files        1814     1814              
  Lines      120475   120441      -34     
  Branches    18526    18524       -2     
==========================================
- Hits        65367    65317      -50     
- Misses      49889    49901      +12     
- Partials     5219     5223       +4     

Flag	Coverage Δ
unittests	54.23% <86.95%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...monMain/kotlin/com/soywiz/korge/view/ShadedView.kt	5.26% <ø> (-1.64%)	⬇️
...otlin/com/soywiz/korge/view/filter/ShaderFilter.kt	93.50% <ø> (-0.09%)	⬇️
...m/soywiz/korge/view/vector/GpuShapeViewCommands.kt	77.34% <0.00%> (+0.07%)	⬆️
...m/soywiz/korge/view/vector/GpuShapeViewPrograms.kt	68.57% <ø> (-0.45%)	⬇️
...mmonMain/kotlin/com/soywiz/korge/view/BlendMode.kt	83.33% <60.00%> (+0.52%)	⬆️
...n/kotlin/com/soywiz/korge/render/BatchBuilder2D.kt	69.90% <100.00%> (-0.88%)	⬇️
...Main/kotlin/com/soywiz/korge/view/fast/FSprites.kt	88.19% <100.00%> (-0.29%)	⬇️
...Test/kotlin/com/soywiz/korge/view/BlendModeTest.kt	100.00% <100.00%> (ø)
.../commonTest/kotlin/com/soywiz/korau/mod/MODTest.kt	75.00% <0.00%> (-25.00%)	⬇️
.../kotlin/com/soywiz/korau/format/AudioFormatTest.kt	88.00% <0.00%> (-8.00%)	⬇️
... and 30 more

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