Fix for PNG predictor issues. Fixes #313.

[gunnsth]

No description provided.

[codecov]

Codecov Report

Merging #320 into v3 will increase coverage by 0.11%.
The diff coverage is 89.58%.

@@            Coverage Diff             @@
##               v3     #320      +/-   ##
==========================================
+ Coverage   38.56%   38.68%   +0.11%     
==========================================
  Files         121      122       +1     
  Lines       23079    23083       +4     
==========================================
+ Hits         8901     8930      +29     
+ Misses      12897    12872      -25     
  Partials     1281     1281

Impacted Files	Coverage Δ
pdf/core/encoding.go	38.56% <87.87%> (+1.89%)	⬆️
pdf/core/paeth.go	93.33% <93.33%> (ø)

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[peterwilliams97]

The changes look good to me.
The code works with some tests from a command line example program. I will convert these tests to a go test and make a PR after this branch is merged.

[peterwilliams97]

The changes look good to me.
The code works with some tests from a command line example program. I will add these tests to your go test file and make a PR after this branch is merged.