Separate image and background color values

[ickshonpe]

Objective

• It's confusing that the BackgroundColor component is also used to control image color.
• Padding for ImageNodes isn't implemented atm, but when it is, BackgroundColor could be used to set the color of the padded area.
• At the moment you have to nest a transparent image inside of another node to give it a background color.
• If a texture doesn't load at the moment, nothing is drawn. With a separate background color, you could fill the node with a bright color to indicate a missing texture.

Solution

Add a color field to UiImage and use it instead of BackgroundColor for image colors.
Draw separate background and image rects for each node, if the respective components are present.

Changelog

* added a `color` field to `UiImage`
* added a manual impl of `Default` for `ImageBundle`
* changed default `background_color` of `ImageBundle` to `Color::NONE`
* changed extract_uinodes to queue separate rects for the background color and the image

Migration Guide

To set the color tint of an image, instead of using the BackgroundColor component use the color field of UiImage.

[ickshonpe]

Another motivation for this is to support forced aspect ratio images with letterboxing when they can't be made to fit into the UI layout (something I implemented in bevy_ui_exact_image). You'd use background_color to specify a color for the letterbox area.

[ChristopherBiscardi]

I believe #11165 included the changes that this PR would've addressed.