Make decoder's Image an option

Instead all methods with mutable self will initialize it on-demand.
There are only few methods that take `&self` and can not do that. For
those that have no error/option return we panic but both of these could
panic previously already. This should be fine by documentation alone. In
particular, the image size can be queried safely.

src/decoder/image.rs

@@ -155,32 +155,6 @@ pub(crate) struct ReadoutLayout {
 }
 
 impl Image {
-    pub(crate) fn no_image() -> Image {
-        Image {
-            width: 0,
-            height: 0,
-            bits_per_sample: 1,
-            samples: 1,
-            extra_samples: vec![],
-            photometric_samples: 1,
-            sample_format: SampleFormat::Uint,
-            photometric_interpretation: PhotometricInterpretation::BlackIsZero,
-            compression_method: CompressionMethod::None,
-            jpeg_tables: None,
-            predictor: Predictor::None,
-            chunk_type: ChunkType::Strip,
-            planar_config: PlanarConfiguration::Chunky,
-            strip_decoder: None,
-            tile_attributes: None,
-            chunk_offsets: Vec::new(),
-            chunk_bytes: Vec::new(),
-            chroma_subsampling: (2, 2),
-            fill_order: 1,
-            color_map: None,
-            decompression_to_host_endian: false,
-        }
-    }
-
     pub fn from_reader<R: Read + Seek>(
         decoder: &mut ValueReader<R>,
         ifd: &Directory,
@@ -528,6 +502,17 @@ impl Image {
         })
     }
 
+    pub(crate) fn check_chunk_type(&self, expected: ChunkType) -> TiffResult<()> {
+        if expected != self.chunk_type {
+            return Err(TiffError::UsageError(UsageError::InvalidChunkType(
+                expected,
+                self.chunk_type,
+            )));
+        }
+
+        Ok(())
+    }
+
     pub(crate) fn colortype(&self) -> TiffResult<ColorType> {
         let is_alpha_extra_samples = matches!(
             self.extra_samples.as_slice(),