Reader: Layers and Frames are not properly linked

[StarArawn]

From reading the spec frames and layers should be linked together. Looking at the code this is kind of the case via the image_for_frame but if you want to get a specific layer's frame data its not possible. Suggested API:

impl<'a> AsepriteFrameRange<'a> {
    pub fn get_image_for_layer<N: AsRef<str>>(&self, layer_name: N) {
        ...
    }
}