### 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)

}
		}
		terrain_bits_meshes_dirty = false;
	}
	int terrain_set = p_tile_data->get_terrain_set();
	if (terrain_set < 0) {
		return;
	}
	TileSet::TerrainMode terrain_mode = get_terrain_set_mode(terrain_set);
	RenderingServer::get_singleton()->canvas_item_add_set_transform(p_canvas_item->get_canvas_item(), p_transform);
	int terrain_id = p_tile_data->get_terrain();
	if (terrain_id >= 0) {
		Color color = get_terrain_color(terrain_set, terrain_id);
		color.a = TERRAIN_ALPHA;
		p_canvas_item->draw_mesh(terrain_meshes[terrain_mode], Ref<Texture2D>(), Transform2D(), color);
	}
	for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
		CellNeighbor bit = CellNeighbor(i);
		if (is_valid_terrain_peering_bit(terrain_set, bit)) {
			terrain_id = p_tile_data->get_terrain_peering_bit(bit);
			if (terrain_id >= 0) {
				Color color = get_terrain_color(terrain_set, terrain_id);
				color.a = TERRAIN_ALPHA;
				p_canvas_item->draw_mesh(terrain_peering_bits_meshes[terrain_mode][bit], Ref<Texture2D>(), Transform2D(), color);
			}
		}
	}
	RenderingServer::get_singleton()->canvas_item_add_set_transform(p_canvas_item->get_canvas_item(), Transform2D());
}
Vector<Vector<Ref<Texture2D>>> TileSet::generate_terrains_icons(Size2i p_size) {