Image garbage collection and on-demand loading

[dhardy]

As of #187, multiple images in multiple atlases are supported and paths are deduplicated. Images are loaded immediately (even if never used), and are only freed if manually cleared (which might not happen; drop handlers can't do so automatically).

On-demand loading

Potentially we can improve this with on-demand loading as follows:

• load_path creates an ID, reads the path only to find the image size, and creates a mapping from the ID to the path and size
• when creating a render queue using a not-loaded image ID, use the cached size to allocate to an atlas, and queue loading the image in a background thread
• in images::Pipeline::prepare block until the image is loaded

Caveat: we end up reading from all used images twice. This may be worse than simply loading them?

Variant: add a hint in load_path controlling whether immediate loading happens.

GC / auto freeing

Given the above, we could implement some garbage collector, e.g. by adding frame-number-of-last-use to each image, to automatically free unused images.

[dhardy]

See also #44