More convenient Encoder API

[RunDevelopment]

This PR makes the Encoder API easier to use with 2 changes:

• Automatic mipmap generation is now enabled by default. (breaking change)
• Encoder::new_image is a new factory method to create an encoder for an image without going through the Header API.

Together, the most common use case (saving a single DDS image with optional mipmaps) has become 3 lines of code:

let mut encoder = Encoder::new_image(writer, image.size, format, mipmaps)?;
encoder.write_surface(image)?;
encoder.finish()?;

This also makes it easier to teach how to use the Encoder API. I can do more progressive disclosure, since the common case no longer involves manually creating a Header and automatic mipmap generation just does the right thing by default. If users need more control, they can dive deeper into the API at their own pace.