Better image aspect ratio handling

[nicopap]

One interesting property of bevy-lunex is that it avoids at all costs breaking image aspect ratio.

I'm not sure how it does it, but it's something cuicui_layout is bad at keeping.

Currently, only images spawned as leaf nodes preserve aspect ratio, when one of the two axis is marked as LeafRule::Content, it will delegate to the content_sized module the computation of this axis, when constraint on the other is declared.

Current cuicui_layout limitations:

• Images used as "root" will stretch to fit, we'd like to "crop" the image (similarly to the bevy ScalingMode)
• Images used as background for container nodes will always stretch to fit the container, and there is no way to constraint the container to a good aspect ratio for its background image.
• A way to work around this is to use ninepatch. In fact, the bevy-lunex cyberpunk example would benefit from ninepatch as details get discarded when the screen aspect ratio changes.

I'm really not in favor of adding a new Rule type for this. I want to make the logic for layouting as minimalist as possible, to make it possible to reason about it.

I think it's something that could be solved in a downstream crate (cuicui_layout_bevy_ui or/and cuicui_layout_bevy_sprite)

^^ I changed opinion on this. After writing a tutorial on cuicui_layout, I'm convinced that an explicit way to keep aspect ratio is ideal. It enables an homogeneous interpretation (and source!) as to how a specific node's size is computed. I think a new set of rules is the best approach here.

• See also More Rules. #97