TextureAtlasBuilder padding

[ickshonpe]

Objective

TextureAtlas supports pregenerated texture atlases with padding, but TextureAtlasBuilder can't add padding when it creates a new atlas.

fixes #8150

Solution

Add a method padding to TextureAtlasBuilder that sets the amount of padding to add around each texture.

When queueing the textures to be copied, add the padding value to the size of each source texture. Then when copying the source textures to the output atlas texture subtract the same padding value from the sizes of the target rects.

unpadded:

padded:

---

Changelog

TextureAtlasBuilder

• Added support for building texture atlases with padding.
• Adds a padding method to TextureAtlasBuilder that can be used to set an amount of padding to add between the sprites of the generated texture atlas.

[github-actions]

Example alien_cake_addict failed to run, please try running it locally and check the result.

[alice-i-cecile]

LGTM once we clarify the docs: the field docs are great, but aren't publicly exposed so it's helpful to repeat them.

[ickshonpe]

It might also be good to expand the texture atlas builder example in another PR.
We could generate atlases with and without padding and render the textures with and without anti-aliasing to demonstrate how different approaches can deal with bleeding issues etc.