PNG: always strip the existing iCCP chunk (backport #2254)

[kmilos]

Backports #2254

[codecov]

Codecov Report

Merging #2256 (e79dc07) into 0.27-maintenance (0f1686f) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@                 Coverage Diff                  @@
##           0.27-maintenance    #2256      +/-   ##
====================================================
- Coverage             58.88%   58.88%   -0.01%     
====================================================
  Files                   149      149              
  Lines                 23208    23203       -5     
  Branches              12716    12711       -5     
====================================================
- Hits                  13666    13662       -4     
  Misses                 6702     6702              
+ Partials               2840     2839       -1     

Impacted Files	Coverage Δ
src/pngimage.cpp	59.84% <50.00%> (-0.27%)	⬇️

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[kmilos]

Thanks!