Allow importing large images

[ShaddyDC]

Currently, importing a glTF file with a large image (eg using gltf::import) fails with the error Image(Limits(LimitError { kind: InsufficientMemory })).
This is because, AFAICT, there is a hard limit of 512MB that an allocator may use for images.

More specifically, the call stack looks something like this:

gltf::import("my.glb")
import_path
import_impl
import_images
image::Data::from_source
image_crate::load_from_memory_with_format
free_functions::load
super::Limits::default

with

impl Default for Limits {
    fn default() -> Limits {
        Limits {
            max_image_width: None,
            max_image_height: None,
            max_alloc: Some(512 * 1024 * 1024),
            _non_exhaustive: (),
        }
    }
}

The solution seems to be to instead use a decoder with limits.
I am not sure what an ideal interface to expose this here would be. I think I would like something like image::Data::from_source_with_limits, maybe?