Refactor cycle and IFD tracking

The previous strategy could not keep track of all cycles such as
reflexive ones at the very start. Additionally, it would fail to be able
to iterate after seeking since it was overly cautious and verified the
next IFD position before allowing the read of a current IFD.

The new implementation is furthermore proof against having multiple
independent chains of IFD which may occur when iterating SubIFDs for
thumbnails. (Note that we do not verify the tree-structure, only a
freedom from cycles).

tests/subsubifds.rs

@@ -1,4 +1,4 @@
-use tiff::{decoder::Decoder, tags::IfdPointer};
+use tiff::decoder::Decoder;
 
 // This file has the following IFD structure:
 //
@@ -20,14 +20,21 @@ fn decode_seek_chain() {
     let file = File::open(TEST_IMAGE_SUBIFD).expect("Cannot open test image");
     let mut decoder = Decoder::new(file).expect("Invalid format to create decoder");
 
+    // Remember the offsets in the first iteration.
+    let offset0 = decoder.ifd_pointer().expect("First IFD pointer not found");
     decoder.next_image().unwrap();
+    let offset1 = decoder.ifd_pointer().expect("Second IFD pointer not found");
     decoder.next_image().unwrap();
+    let offset2 = decoder.ifd_pointer().expect("Third IFD pointer not found");
     assert!(!decoder.more_images());
 
     // Ensure we can seek, to do this again.
     decoder.seek_to_image(0)
         .expect("Failed to seek to first image");
+    assert_eq!(Some(offset0), decoder.ifd_pointer());
     decoder.next_image().unwrap();
+    assert_eq!(Some(offset1), decoder.ifd_pointer());
     decoder.next_image().unwrap();
+    assert_eq!(Some(offset2), decoder.ifd_pointer());
     assert!(!decoder.more_images());
 }