Function azul_core::app_resources::build_add_image_resource_updates

pub fn build_add_image_resource_updates<T: FontImageApi>(
    app_resources: &AppResources, 
    render_api: &mut T, 
    pipeline_id: &PipelineId, 
    images_in_dom: &FastHashSet<ImageId>, 
    image_source_load_fn: LoadImageFn
) -> Vec<(ImageId, AddImageMsg)>

Given the images of the current frame, returns AddImages of which image keys are currently not in the current_registered_fonts and need to be added. Modifies last_frame_image_keys to contain the added image keys

Deleting images can only be done after the entire frame has finished drawing, otherwise (if removing images would happen after every DOM) we'd constantly add-and-remove images after every IFrameCallback, which would cause a lot of I/O waiting.