Tiff: Add maximum IFD threshold

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

This PR adds a maximum of 65534 IFD directories. Images with more then that will throw a ImageFormatException.
Fixes #1891

[codecov]

Codecov Report

Merging #1892 (f45e184) into master (53456a5) will increase coverage by 0%.
The diff coverage is 80%.

@@          Coverage Diff           @@
##           master   #1892   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         957     957           
  Lines       50389   50391    +2     
  Branches     6263    6264    +1     
======================================
+ Hits        44176   44180    +4     
+ Misses       5181    5180    -1     
+ Partials     1032    1031    -1     

Flag	Coverage Δ
unittests	87% <80%> (+<1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Tiff/Ifd/DirectoryReader.cs	93% <75%> (+4%)	⬆️
src/ImageSharp/Formats/Tiff/Ifd/EntryReader.cs	85% <100%> (ø)
src/ImageSharp/Formats/Tiff/TiffThrowHelper.cs	28% <0%> (+14%)	⬆️

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[antonfirsov]

Looks good to me, hope this is the only place that has to be changed.

/cc @IldarKhayrutdinov if you have a moment.

[IldarKhayrutdinov]

good 👍

[JimBobSquarePants]

@brianpopow Great job dealing with this.!