Fix Letter's PointSize computation

[BobLd]

• fix Letter's PointSize computation by applying the transform to a rectangle of height fontSize
• add test with rotated letters

Issue was related to https://stackoverflow.com/questions/48010235/pdf-specification-get-font-size-in-points

[codecov-io]

Codecov Report

Merging #229 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
- Coverage   64.51%   64.49%   -0.03%     
==========================================
  Files         600      600              
  Lines       39675    39666       -9     
==========================================
- Hits        25596    25582      -14     
- Misses      14079    14084       +5     

Impacted Files	Coverage Δ
...UglyToad.PdfPig/Graphics/ContentStreamProcessor.cs	86.87% <100.00%> (-0.26%)	⬇️
src/UglyToad.PdfPig.Core/TransformationMatrix.cs	73.03% <0.00%> (-2.46%)	⬇️

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[InusualZ]

Since you are fixing the calculation, you may want to fix the description too here

[BobLd]

well spotted

[EliotJones]

I assume this produces the correct results, thanks for fixing it :)