Make Decoder parts public so they can be used

I should maybe set the properties to be private and accessible strictly
through accessor methods. That way nobody can accidentally write into
the values. Oh well, that's a thing for later.

src/decoder.rs

@@ -57,12 +57,12 @@ mod codec_utils {
 }
 
 
-struct Decoder<I: Iterator<Item = u8>> {
+pub struct Decoder<I: Iterator<Item = u8>> {
     // Image metadata
-    width: u32,
-    height: u32,
-    channels: u8,
-    colorspace: u8,
+    pub width: u32,
+    pub height: u32,
+    pub channels: u8,
+    pub colorspace: u8,
 
     // QOI codec state information
     back_buffer: [PixelRGBA; 64],
@@ -76,7 +76,7 @@ impl<I> Decoder<I>
 where
     I: Iterator<Item = u8>,
 {
-    fn try_new(mut bytes: I) -> Result<Self, DecodeError> {
+    pub fn try_new(mut bytes: I) -> Result<Self, DecodeError> {
         let _ = try_read_magic(&mut bytes)?;
         Ok(Self {
             width: try_read_u32(&mut bytes)?,