Regression: rendered texture not updating after change to Image

## Bevy version

0.15.1

## What you did

Minimal repro

```rust
use bevy::{
asset::RenderAssetUsages,
color::palettes::css,
prelude::*,
render::render_resource::{Extent3d, TextureDimension, TextureFormat},
};

fn main() {
App::new()
.add_plugins(DefaultPlugins)
.add_systems(Startup, init)
.add_systems(FixedUpdate, draw)
.run();
}

fn init(
mut commands: Commands,
mut images: ResMut>,
mut materials: ResMut>,
mut meshes: ResMut>,
) {
let image = Image::new_fill(
Extent3d {
width: 256,
height: 256,
depth_or_array_layers: 1,
},
TextureDimension::D2,
&(css::REBECCA_PURPLE.to_u8_array()),
TextureFormat::Rgba8UnormSrgb,
RenderAssetUsages::MAIN_WORLD | RenderAssetUsages::RENDER_WORLD,
);
let image_handle = images.add(image);
let mesh = meshes.add(Rectangle::new(256., 256.));
let material = materials.add(image_handle);

commands.spawn((Camera2d, Mesh2d(mesh), MeshMaterial2d(material)));
}

fn draw(
mut images: ResMut>,
material: Single<&MeshMaterial2d>,
mut materials: ResMut>,
) {
let Some(m) = materials.get(*material) else {
return;
};

let Some(texture) = &m.texture else {
return;
};

let Some(img) = images.get_mut(texture) else {
return;
};

for x in 0..50 {
for y in 0..50 {
let _ = img.set_color_at(x, y, Color::WHITE);
}
}
// This call is required to update render.
materials.get_mut(*material);
}
```

Create an `Image`. Add it to a `MeshMaterial2d`, and render the material using a `Mesh2d`. In the `FixedUpdate` schedule, modify the `Image` asset.

## What went wrong

The changes will not be rendered unless a call is made to `materials.get_mut(&handle)` _after_ the changes have been made.

## Additional information

It appears this may have been "re-broken" for quite awhile after being fixed in https://github.com/bevyengine/bevy/commit/107dd736874d07d6c79d0c44ce6ee9763aed9a12. It's also possible that this is "working as intended" in the retained rendering world we live in, but I'm not flash enough to understand all the possible implications!

For some reason this behaviour is not evident in `Update`, only `FixedUpdate`. I have no idea why! I can also make it manifest in `Update` if I wait on mouse input e.g. by "drawing" on the image using the cursor, which smacks of a timing issue but no clue what.