Update ASSETS_LOADED one frame later so users can skip the placeholder.

[yaymukund]

When textures are rendered, there is a placeholder image that flashes for a single frame before the actual image. Here is an example of how it looks:

Example reproduction:

impl GameLoop for Game {
    fn new(_c: &mut EngineState) -> Self {
        load_multiple_textures(
            // Includes the "monkey" texture.
            crate::constants::TEXTURES
                .iter()
                .map(|(a, b)| (a.to_string(), b.to_string()))
                .collect_vec(),
        );

        clear_background(BLACK);
        Self
    }

    fn update(&mut self, c: &mut EngineContext) {
        if assets_loaded() < assets_queued_total() {
            println!("{}/{} loaded", assets_loaded(), assets_queued_total());
            return;
        }

        draw_sprite(
            texture_id("monkey"),
            vec2(5.0, 0.0),
            WHITE,
            0,
            vec2(2.60, 3.15),
        );
    }
}

Here's my attempt to track it down:

1. run_early_update_stages() processes the asset queues...
2. ...and then spawns an async task that does inc_assets_loaded() saying it is "loaded." It is not actually loaded yet.
3. The renderer update can't find the texture and instead defaults to the "error" texture id, which is very similar to that image I linked above.
4. Sometime between the renderer update and the next frame, the texture gets loaded. I am still not sure where this hapens.

Fix in this branch. We just put off the rendering by 1 frame.

[yaymukund]

This doesn't seem to fix things for Sprite::new