Allow non-chained directories in writer

This is just the implementation details. In the DirectoryEncoder we did
not differentiate between different directories that should go into the
main image chain and other directories. This is now possible by storing
the prior offset as an explicit Option<NonZeroU64>.

Additionally, the whole encoder did not allow any intermediate data
between its last directory and new directories on the chain. The offset
of the relevant `next` field was not stored explicitly but derived from
the current stream position at the moment of starting the directory.
This would not allow any padding data or data written outside the
returned `DirectoryEncoder` object.

This relaxed both requirements by turning the offsets into attributes.

src/encoder/writer.rs

@@ -69,6 +69,10 @@ impl<W: Write> TiffWriter<W> {
         self.offset
     }
 
+    pub(crate) fn previous_ifd_pointer<K: super::TiffKind>(&self) -> u64 {
+        self.offset() - core::mem::size_of::<K::OffsetType>() as u64
+    }
+
     pub fn last_written(&self) -> u64 {
         self.byte_count
     }