[Core] Make ImageFormatLoader extensible.

[Faless]

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Make ImageFormatLoader RefCounted, expose it to extensions. There is an AVIF implementation (WIP) here: https://github.com/Faless/gd-avif/releases/tag/v0.0.1-godot-pr

Proposals to support esoteric image format: godotengine/godot-proposals#2461 godotengine/godot-proposals#3726

Draft status:

• Is loader.add_format_loader okay?
• Should we add class_name scripts like done in ResourceFormatLoader?
• Should we add ResourceFormatLoader.add_custom_image_format_loader(String script_path);/ResourceFormatLoader.add_custom_image_format_loader_class(StringName p_class); (class_names and extensions?)

[fire]

I have MaterialX module that requires a StandardMaterial3D loader, but that's related but offtopic.

[reduz]

I guess this needs the FileAccess bindings for this to work properly

[Faless]

Blocked by #65271

[Faless]

Blocked by #65271

Rebased on it, updated the PR and the avif module (not tagged yet).

[Faless]

Should be good for review, let me know if you prefer it to be exposed as a separate ImageLoaderExtension class (CC @reduz ).

[akien-mga]

Thanks!