into_png: Consume a Pixmap to write it to PNG without copying

Nobody has claimed this yet.

The TODO note in encode_png to eliminate the clone of the Pixmap can be addressed by adding a variant function that consumes `self`. Here's an example I've implemented from the outside:
```rust
pub fn into_png(mut image: MaybeFromPool) -> Result, png::EncodingError> {
for pixel in image.pixels_mut() {
unsafe {
// Treat this PremultipliedColorU8 slice as a ColorU8 slice
*pixel = mem::transmute(pixel.demultiply());
}
}

let mut data = Vec::with_capacity(1024 * 1024);
{
let mut encoder = png::Encoder::new(&mut data, image.width(), image.height());
encoder.set_color(png::ColorType::Rgba);
encoder.set_depth(png::BitDepth::Eight);
let mut writer = encoder.write_header()?;
writer.write_image_data(image.data())?;
}

Ok(data)
}
```

Locate encode_png and its TODO, then read the surrounding MaybeFromPool and Pixmap ownership APIs. Use the issue's example as the behavior reference and verify that the consuming PNG path produces the same output without cloning the Pixmap.

Written by the indexing model from the issue text.