add infrastructure to font to retrieve character bounds, this is unused and will throw for now. add format 4 cmap subtable for truetype cmap table

src/UglyToad.PdfPig/Fonts/IFont.cs

@@ -19,6 +19,8 @@
 
         decimal GetWidth(int characterCode);
 
+        PdfRectangle GetBoundingBox(int characterCode);
+
         TransformationMatrix GetFontMatrix();
     }
 }