Scale alpha planes [0, (1 << bit-depth) - 1] to [0, 65536]

[ledyba-z]

I found that I forgot to scale alpha planes.

In 10bit and 12bit images, alpha plane contains values which ranged from 0 to 1023(10bit) or 4095(12bit).

In this PR, I scale them to 0 to 65535.

[ledyba-z]

Thanks!

[codecov]

Codecov Report

Merging #14 into master will decrease coverage by 0.1%.
The diff coverage is 83.72%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   86.19%   86.09%   -0.11%     
==========================================
  Files           4        4              
  Lines         971     1014      +43     
==========================================
+ Hits          837      873      +36     
- Misses        134      141       +7

Impacted Files	Coverage Δ
SDWebImageAVIFCoder/Classes/SDImageAVIFCoder.m	86.47% <83.72%> (-0.13%)	⬇️

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