Images: support multiple texture atlases; use common bind group

[dhardy]

Image atlases are now created on demand, thus no atlases if no images are used, and new ones allocated as necessary to hold loaded images.

Limitations:

• images are currently only freed if manually cleared (I will create a new issue for garbage collection)
• atlases are never freed
• multiple uses of the same path are not deduplicated
• images too big to fit in an atlas are not handled (ImageError::Allocation)
• images are loaded immediately on request, where they could be background loaded