PNG Identify reads zTXt, iTXt and exif cunks

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This PR changes PNG Identify, so it will not ignore EXIF or zTXt, iTXt chunks.

[codecov]

Codecov Report

Merging #1187 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1187      +/-   ##
==========================================
+ Coverage   82.50%   82.53%   +0.02%     
==========================================
  Files         692      692              
  Lines       29974    29983       +9     
  Branches     3389     3390       +1     
==========================================
+ Hits        24731    24746      +15     
+ Misses       4543     4540       -3     
+ Partials      700      697       -3     

Flag	Coverage Δ
#unittests	82.53% <100.00%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Png/PngDecoderCore.cs	90.65% <100.00%> (+1.41%)	⬆️

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[JimBobSquarePants]

Well spotted!