Thin Lines between grids in grid display (bevy UI)

## Bevy version

0.15.1

## \[Optional\] Relevant system information

```ignore
`AdapterInfo { name: "NVIDIA GeForce RTX 4080", vendor: 4318, device: 9988, device_type: DiscreteGpu, driver: "NVIDIA", driver_info: "560.94", backend: Vulkan }`
```

## What you did

```rust
commands.spawn(Node {
width: Val::Percent(100.),
height: Val::Percent(100.),
position_type: PositionType::Absolute,
justify_content: JustifyContent::Center,
align_items: AlignItems::Center,
..default()
}).with_children(|parent| {
parent.spawn(
Node {
display: Display::Grid,
width: Val::Percent(50.),
height: Val::Percent(50.),
grid_template_rows: RepeatedGridTrack::flex(3, 1.0),
grid_template_columns: RepeatedGridTrack::flex(3, 1.0),
..default()
}
).with_children(|parent| {

for _ in 0..3 {
for _ in 0..3 {
parent.spawn((
Node::default(),
BackgroundColor::from(Color::WHITE),
));
}
}
});
});
```

## What went wrong

Thin lines seem to appear between the grid cells in the Grid Display Layout when I resize the window.



Start by running the supplied Rust example on Bevy 0.15.1 and resize the window to reproduce the lines between the 3×3 grid cells. Trace the Bevy UI grid layout and rendering behavior involved in that example. Done means the white grid cells remain visually continuous without thin lines while resizing.

Written by the indexing model from the issue text.