ImageButton does not use y value of style.spacing.button_padding

**Describe the bug**
ImageButton only uses the x value of `ui.style().spacing.button_padding` when drawn. In practice this means that ImageButtons cannot have independent padding on X / Y, and the Y value doesn't impact the layout of the frame.

[Link to code: egui/src/widgets/button.rs](https://github.com/emilk/egui/blob/9faf4b44ff12af00c26763667fdf41bf64a7c060/crates/egui/src/widgets/button.rs#L627C24-L627C25)
```rust
// egui/src/widgets/button.rs

impl<'a> Widget for ImageButton<'a> {
fn ui(self, ui: &mut Ui) -> Response {
let padding = if self.frame {
// so we can see that it is a button:
Vec2::splat(ui.spacing().button_padding.x) // <--- this line
} else {
Vec2::ZERO
};
...
```

**To Reproduce**
Steps to reproduce the behavior:
1. Change the `ui.style_mut().spacing.button_padding` to a Vec2 with different X & Y values.
2. Create an ImageButton.
3. Your ImageButton's padding will only use the value in X.

**Expected behavior**
ImageButton padding's Y value can be set independently of its X value.

**Screenshots**
A = `ui.style_mut().spacing.button_padding = egui::Vec2::new(2.0, 2.0);`
B = `ui.style_mut().spacing.button_padding = egui::Vec2::new(8.0, 2.0);`
C = `ui.style_mut().spacing.button_padding = egui::Vec2::new(2.0, 8.0);`
D = `ui.style_mut().spacing.button_padding = egui::Vec2::new(2.0, 2.0);`

**Desktop (please complete the following information):**
- OS: Windows 10
- Platform: Bevy, using bevy_egui 0.23.0/0.24.0. Also tested on pure egui off master as of 18 Jan 2024

**Additional context**
The fix seems as simple as changing this line:
```rust
Vec2::splat(ui.spacing().button_padding.x)
```
to
```rust
ui.spacing().button_padding
```
But I don't know if there was a specific reason it uses splat now. [Looks like this change was made a few years ago.](https://github.com/emilk/egui/commit/a5a5d6176d6210eed19e9d6e5966d4c04749d9f6)

Start in crates/egui/src/widgets/button.rs at the ImageButton implementation and inspect how button_padding is used for layout. Reproduce the issue with distinct X and Y spacing values, then verify that the ImageButton responds independently to both axes.

Written by the indexing model from the issue text.