Sample types in layouts, resize DecoderResult

By adding the type interpretation to `BufferLayoutPreference` (simply
computed from the same fields that are trivial to fetch in
`ReadoutLayout` with the rest) we can allocate an appropriate
`DecoderResult` without any `Decoder` instance. We still want a Limits
instance though. This lets a caller read _typed_ data from the `_bytes`
methods which perform less allocations. It also lets them read multiple
planes in a more convenient interface. See, `read_image` will keep its
planar treatment bug for a little bit so you don't accidentally break
during an upgrade.

This also implies less code is being monomorphized, just a bonus.

examples/decode.rs

@@ -10,12 +10,17 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     let io = std::io::BufReader::new(file);
     let mut reader = Decoder::new(io)?;
 
+    let mut data = DecodingResult::I8(vec![]);
+    let limits = tiff::decoder::Limits::unlimited();
+
     for i in 0u32.. {
         let layout = reader.image_buffer_layout()?;
         let colortype = reader.colortype()?;
         let dimensions = reader.dimensions()?;
 
-        let mut data = reader.read_image()?;
+        data.resize_to(&layout, &limits)?;
+        reader.read_image_bytes(data.as_buffer(0).as_bytes_mut())?;
+
         debug_planes(i, &mut data, &layout, &colortype, dimensions);
 
         if !reader.more_images() {