### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

Ref<Texture2D> src = src_canvas_texture->get_diffuse_texture();
		Ref<ImageTexture> image_texture;
		if (src.is_valid()) {
			padded_texture->set_diffuse_texture(_create_padded_image_texture(src));
		}
		
		src = src_canvas_texture->get_normal_texture();
		if (src.is_valid()) {
			padded_texture->set_normal_texture(_create_padded_image_texture(src));
		}
		
		src = src_canvas_texture->get_specular_texture();
		if (src.is_valid()) {
			padded_texture->set_specular_texture(_create_padded_image_texture(src));
		}
		
		padded_texture->set_specular_color(src_canvas_texture->get_specular_color());
		padded_texture->set_specular_shininess(src_canvas_texture->get_specular_shininess());
		padded_texture->set_texture_filter(src_canvas_texture->get_texture_filter());
		padded_texture->set_texture_repeat(src_canvas_texture->get_texture_repeat());
	} else {
		
		Ref<ImageTexture> image_texture = _create_padded_image_texture(texture);
		padded_texture->set_diffuse_texture(image_texture);
	}
	padded_texture->connect_changed(callable_mp(this, &TileSetAtlasSource::_queue_update_padded_texture));
	emit_changed();
}
void TileSetScenesCollectionSource::_compute_next_alternative_id() {