Fix issue calculating the stripIndex for planar tiff's

[brianpopow]

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Description

Fixes a bug calculating the stripIndex for planar tiff images.

example strip offsets and byte counts from flower-rgb-planar-08-6strips.tiff (tiffinfo -s)

6 Strips:
            offsets, byte count
      0: [       8,     2190]
      1: [    2198,      949]
      2: [    3147,     2190]
      3: [    5337,      949]
      4: [    6286,     2190]
      5: [    8476,      949]

@IldarKhayrutdinov could you please review this change when you have time?

[codecov]

Codecov Report

Merging #1723 (4f19692) into master (83427f2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1723   +/-   ##
=======================================
  Coverage   84.36%   84.36%           
=======================================
  Files         822      822           
  Lines       36022    36023    +1     
  Branches     4204     4204           
=======================================
+ Hits        30391    30392    +1     
  Misses       4816     4816           
  Partials      815      815           

Flag	Coverage Δ
unittests	84.36% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Tiff/TiffDecoderCore.cs	88.70% <100.00%> (+0.09%)	⬆️

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[IldarKhayrutdinov]

@brianpopow yeah, it seems there was a bug, now looks good!

[brianpopow]

yeah, it seems there was a bug, now looks good!

@IldarKhayrutdinov ok, thanks for taking a look!

[IldarKhayrutdinov]

new test files are good,
we needs to add more test files for tiff

[brianpopow]

we had test files for planar, but only with 3 strips, so the bug was not triggered.