New Decoder creation function, more metadata

The qoi_header is specified as knowing the width, height, channels, and
colorspace. I figure this information would be captured by the decoder,
if anywhere. I've added more properties to the struct, and created a
new, fallible construction function.

src/decoder.rs

@@ -35,6 +35,7 @@ fn try_read_u32<I: Iterator<Item=u8>>(bytes: &mut I) -> Result<u32, DecodeError>
 }
 
 mod codec_utils {
+    use super::DecodeError;
     use super::PixelRGBA;
     pub(crate) fn hash(pixel: PixelRGBA) -> u8 {
         pixel
@@ -45,10 +46,24 @@ mod codec_utils {
             .wrapping_add(pixel.a.wrapping_mul(11))
             % 64
     }
+
+    pub (crate) fn read_u8_rewrap<I: Iterator<Item = u8>>(bytes: &mut I) -> Result<u8, DecodeError> {
+        if let Some(byte) = bytes.next() {
+            return Ok(byte);
+        } else {
+            return Err(DecodeError::EarlyIteratorExhaustion);
+        }
+    }
 }
 
 
 struct Decoder<I: Iterator<Item = u8>> {
+    // Image metadata
+    width: u32,
+    height: u32,
+    channels: u8,
+    colorspace: u8,
+
     // QOI codec state information
     back_buffer: [PixelRGBA; 64],
     prev_pixel: PixelRGBA,
@@ -61,6 +76,20 @@ impl<I> Decoder<I>
 where
     I: Iterator<Item = u8>,
 {
+    fn try_new(mut bytes: I) -> Result<Self, DecodeError> {
+        let _ = try_read_magic(&mut bytes)?;
+        Ok(Self {
+            width: try_read_u32(&mut bytes)?,
+            height: try_read_u32(&mut bytes)?,
+            channels: codec_utils::read_u8_rewrap(&mut bytes)?,
+            colorspace: codec_utils::read_u8_rewrap(&mut bytes)?,
+            back_buffer: [PixelRGBA::zero(); 64],
+            prev_pixel: PixelRGBA { r:0, g:0, b:0, a: 255},
+            bytes,
+            run_len: 0,
+        })
+    }
+
     fn new(bytes: I) -> Self {
         Self {
             back_buffer: [PixelRGBA::zero(); 64],