Move Directory from image into decoder proper

This prepares allowing to iterate over a TIFF file with invalid images
while `Image` maintains invariants. For instance we precalculate the
amount of buffer space and verify that the count of strips makes sense
in the constructor—so we do not have to redo that for every other entry
point.

src/decoder/image.rs

@@ -64,7 +64,6 @@ impl TileAttributes {
 
 #[derive(Debug)]
 pub(crate) struct Image {
-    pub ifd: Option<Directory>,
     pub width: u32,
     pub height: u32,
     pub bits_per_sample: u8,
@@ -149,9 +148,9 @@ pub(crate) struct ReadoutLayout {
 impl Image {
     pub fn from_reader<R: Read + Seek>(
         decoder: &mut ValueReader<R>,
-        ifd: Directory,
+        ifd: &Directory,
     ) -> TiffResult<Image> {
-        let mut tag_reader = TagReader { decoder, ifd: &ifd };
+        let mut tag_reader = TagReader { decoder, ifd };
 
         let width = tag_reader.require_tag(Tag::ImageWidth)?.into_u32()?;
         let height = tag_reader.require_tag(Tag::ImageLength)?.into_u32()?;
@@ -435,7 +434,6 @@ impl Image {
         }
 
         Ok(Image {
-            ifd: Some(ifd),
             width,
             height,
             bits_per_sample,