Interpret PHOTOMETRIC_MINISWHITE as Gray

[mkitti]

This sets the interpretation of PHOTOMETRIC_MINISWHITE to Gray

xref: https://discourse.julialang.org/t/problem-in-reading-tiff-floats-file/106610

Fix #133

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.85%. Comparing base (4613107) to head (77aec0f).
Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layout.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
- Coverage   91.95%   91.85%   -0.10%     
==========================================
  Files          13       13              
  Lines         920      921       +1     
==========================================
  Hits          846      846              
- Misses         74       75       +1     

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[tlnagy]

I would be hesitant to merge this PR without JuliaGraphics/ColorTypes.jl#295 being addressed first as we would be violating the Colors.jl promise to faithfully capture the interpretation of the raw data. We could load the data as a standard Gray and ignore the fact that it is supposed to be inverted, but that's inconsistent with how we treat all other interpretations.