feat(tex): add has_opaque_alpha for crunch

[mklokocka]

Add a has_opaque_alpha method for the CrunchTexture class. has_alpha built in functions does not account for actual alpha contents, just the validness of the alpha channel.

Use it in format selection.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.97%. Comparing base (025a891) to head (87a13fd).

Files with missing lines	Patch %	Lines
src/tex/crunch_texture.cpp	80.00%	3 Missing ⚠️
src/tex/optimize.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   78.91%   78.97%   +0.06%     
==========================================
  Files          41       41              
  Lines        1873     1888      +15     
==========================================
+ Hits         1478     1491      +13     
- Misses        395      397       +2     

Files with missing lines	Coverage Δ
include/btu/tex/crunch_texture.hpp	100.00% <ø> (ø)
src/tex/optimize.cpp	58.70% <0.00%> (+0.37%)	⬆️
src/tex/crunch_texture.cpp	76.84% <80.00%> (+0.59%)	⬆️

... and 1 file with indirect coverage changes

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