Auto width/height for manganis images

## Feature Request

You can currently not set the image size for images or set both the width and height. You should be able to set either the width or height and have the other derived from the aspect ratio of the image

## Implement Suggestion

Allow `with_width` or `with_height` and derive the other dimension

```rust
// Resize the image at compile time to make the assets smaller
pub const RESIZED_PNG_ASSET: Asset =
asset!("/assets/image.png", AssetOptions::image().with_width(52));
```

or

```rust
// Resize the image at compile time to make the assets smaller
pub const RESIZED_PNG_ASSET: Asset =
asset!("/assets/image.png", AssetOptions::image().with_height(52));
```