Deprecate encoding::gamma and its content

[Ogeon]

Deprecates Gamma, GammaFn and F2p2 due to being incorrectly implemented and being more error prone than just implementing FromLinear and IntoLinear for a custom type.

The current implementation of GammaFn has the FromLinear and IntoLinear implementations reversed, and fixing that would break code that compensates for the error. Deprecation, together with the extended number of RGB standards added recently, should hopefully provide better alternatives without suddenly changing the visual output of existing uses.

The recommended migration options are to either find a fitting alternative in palette::encoding or implementing FromLinear and IntoLinear for a custom type. Suggestions for more additions are also welcome.

[codspeed-hq]

CodSpeed Performance Report

Merging #427 will degrade performances by 19.68%

_{Comparing deprecate_gamma (9cf298b) with master (234309c)}

Summary

❌ 1 regressions
✅ 46 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	deprecate_gamma	Change
❌	matrix_inverse	352.5 ns	438.9 ns	-19.68%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.69%. Comparing base (234309c) to head (9cf298b).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
+ Coverage   82.63%   82.69%   +0.06%     
==========================================
  Files         131      131              
  Lines       20485    20457      -28     
  Branches    20485    20457      -28     
==========================================
- Hits        16927    16917      -10     
  Misses       3378     3378              
+ Partials      180      162      -18     

Components	Coverage Δ
palette	82.75% <ø> (+0.06%)	⬆️
palette_derive	81.98% <ø> (ø)