Track decode progress and stop at W*H pixels

I was thinking about doing some partial parsing where I retrieve a few
extra bytes to see if it's a footer, but that's really hard with an
iterator as an input stream.

Instead, I'm just going to count how many pixels the decoder has output
and stop iterating at that point.

Now to fix the test cases that don't have an image size assigned.

src/decoder.rs

@@ -70,6 +70,8 @@ pub struct Decoder<I: Iterator<Item = u8>> {
 
     bytes: I,
     run_len: u8,
+    // Counter to ensure we stop after getting to Self::width * Self::height pixels
+    output_count: usize,
 }
 
 impl<I> Decoder<I>
@@ -87,6 +89,7 @@ where
             prev_pixel: PixelRGBA { r:0, g:0, b:0, a: 255},
             bytes,
             run_len: 0,
+            output_count: 0,
         })
     }
 }
@@ -98,6 +101,13 @@ where
     type Item = PixelRGBA;
 
     fn next(&mut self) -> Option<Self::Item> {
+        // count pixels. When we reach WIDTH x HEIGHT, check for footer and quit.
+        if self.output_count == (self.width * self.height) as usize {
+            return None;
+        } else {
+            self.output_count += 1;
+        }
+
         if self.run_len > 0 {
             self.run_len -= 1;
             Some(self.prev_pixel)
@@ -206,6 +216,7 @@ mod test {
                 },
                 bytes,
                 run_len: 0,
+                output_count: 0,
             }
         }
 
@@ -221,6 +232,7 @@ mod test {
                 prev_pixel,
                 bytes,
                 run_len: 0,
+                output_count: 0,
             }
         }
 
@@ -238,6 +250,7 @@ mod test {
                 prev_pixel: PixelRGBA { r: 0, g: 0, b: 0, a: 255 },
                 bytes,
                 run_len: 0,
+                output_count: 0,
             }
         }