Track input stream position on load.

[JimBobSquarePants]

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

We weren't tracking input stream positional offsets when loading which leads to issues. Fix #818

[codecov]

Codecov Report

Merging #820 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
+ Coverage   88.65%   88.65%   +<.01%     
==========================================
  Files        1015     1015              
  Lines       44120    44121       +1     
  Branches     3178     3178              
==========================================
+ Hits        39116    39117       +1     
  Misses       4241     4241              
  Partials      763      763

Impacted Files	Coverage Δ
src/ImageSharp/IO/DoubleBufferedStreamReader.cs	91.04% <100%> (+0.13%)	⬆️

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[codecov]

Codecov Report

Merging #820 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
+ Coverage   88.65%   88.65%   +<.01%     
==========================================
  Files        1015     1015              
  Lines       44120    44121       +1     
  Branches     3178     3178              
==========================================
+ Hits        39116    39117       +1     
  Misses       4241     4241              
  Partials      763      763

Impacted Files	Coverage Δ
src/ImageSharp/IO/DoubleBufferedStreamReader.cs	91.04% <100%> (+0.13%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2849fd9...42ceecf. Read the comment docs.

[tocsoft]

👍 looks good to me.