support gs operator, fix systemfonts, apply rotation to glyphs

- begin adding support for extended graphics state (the 'gs' operator) including setting the font #39.
- apply page level rotation to the glyph bounding box and width to get correct glyph sizes #41.
- wrap page rotation in a value type to ensure the value is restricted to right angle rotations and provide convenience members #42.
- fix bug where system font finder never worked for truetype fonts because it began reading the file from the wrong offset.

src/UglyToad.PdfPig/Graphics/Operations/SpecialGraphicsState/SetGraphicsStateOperatorsFromDictionary.cs

@@ -34,6 +34,7 @@
         /// <inheritdoc />
         public void Run(IOperationContext operationContext)
         {
+            operationContext.SetNamedGraphicsState(Name);
         }
 
         /// <inheritdoc />