Add UiImage offset property

[afonsolage]

Objective

Currently UiImage component is just a unit struct for Handle<Image> . At it's core, it draws the image and that's it. If someone needs to have a more complex use case, like implementing a nine patch, one have to use TextureAtlas which is good, but is a overkill for ui works.

This PR is related to #5070, but it aims to make TextureAtlas to work with UiImage which is a different use case IMO, since one should use TextureAtlas and UiImage together on same entity.

Solution

Add an offset property to UiImage and enable to render a portion of the image, much like a TexturaAtlas but simplified, so users may create it's own image atlas and reuse on many UiImage components.

Changelog

• Added offset property on UiImage and changed to from unit struct to normal struct;
• Removed Deref and DerefMut from UiImage since it's no longer a unit struct;
• Changed extract_uinodes to use UiImage::offset when it's non-zero;
• Added image_button example:
  

Migration Guide

UiImage is now a normal struct so it can't be directly dereferenced anymore and should use normal struct initialization.

[alice-i-cecile]

Ping @ManevilleF for review here :)

[alice-i-cecile]

Suggested change

	// Change image filter to a pixel-art friendly
	// Change image filter to a pixel-art friendly mode

[alice-i-cecile]

Users would really benefit from some more advice on how these values were determined.

[alice-i-cecile]

Suggested change

	// Default image has no offset, but that's OK since it'll be update it on button_system
	// Default image has no offset, but that's OK since it'll be updated in button_system

[alice-i-cecile]

Code quality and docs seem fine, minus a few nits. I'm not fully sold on the grand vision here, so I'll defer to @mockersf and @ManevilleF who've thought about this space more.

[ManevilleF]

The new property offset should be called rect and I think it should be a separate component, overriding the default behaviour to be more ECS friendly.

But to customize Rect of a texture is what a TextureAtlas is for, and I think #5103 + #5072 is the way, as UI and sprites shouldn't behave differently in that scope

[mockersf]

I can't do a complete review anytime soon, but anything that impact rendered size should impact layout, and I didn't see layout files being modified in this PR

[alice-i-cecile]

Closing in favor of #5103; let's collect our efforts there.