`TextureAtlasBuilder`: Return unplaced textures instead of failing

## What problem does this solve or what need does it fill?

`TextureAtlasBuilder` either places all the images it was given within an atlas of the given `max_size` or it fails and does nothing. There's no support for partial completion/failure.

## What solution would you like?

Add an option so that if TextureAtlasBuilder::build fails to fit all images into a single max_size atlas texture, it returns an atlas containing the images that were successfully placed, along with a list of the unplaced images.

Start at TextureAtlasBuilder::build and trace how image placement failure currently aborts atlas creation. Review the surrounding atlas types and existing tests, if present, before deciding how the option exposes placed and unplaced images. Done means a build that cannot fit every image can return the successfully placed atlas together with the images that remain unplaced.

Written by the indexing model from the issue text.