Introduce IBlock and ILettersBlock interfaces (Round 2) (#1263)

* Code review changes
- Keep the Bounds property on the image classes so this isn't a major breaking API change
- Don't expose letters collection

* Minor fix

* Switch to using BoundingBox in the library

---------

Co-authored-by: davmarksman <david@brokit.co.uk>

src/UglyToad.PdfPig.Tests/Integration/GithubIssuesTests.cs

@@ -758,7 +758,7 @@
                 {
                     var letter = page1.Letters[l];
                     Assert.Equal(TextOrientation.Other, letter.TextOrientation);
-                    Assert.Equal(45.0, letter.GlyphRectangle.Rotation, 5);
+                    Assert.Equal(45.0, letter.BoundingBox.Rotation, 5);
                 }
 
                 var page2 = document.GetPage(2);