### 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)

tile_lines_mesh->add_surface_from_arrays(Mesh::PRIMITIVE_LINE_STRIP, a, Array(), Dictionary(), Mesh::ARRAY_FLAG_USE_2D_VERTICES);
		tile_meshes_dirty = false;
	}
	if (p_filled) {
		p_canvas_item->draw_mesh(tile_filled_mesh, p_texture, p_transform, p_color);
	} else {
		p_canvas_item->draw_mesh(tile_lines_mesh, Ref<Texture2D>(), p_transform, p_color);
	}
}
Vector<Point2> TileSet::get_terrain_polygon(int p_terrain_set) {
	if (tile_shape == TileSet::TILE_SHAPE_SQUARE) {
		return _get_square_terrain_polygon(tile_size);
	} else if (tile_shape == TileSet::TILE_SHAPE_ISOMETRIC) {
		return _get_isometric_terrain_polygon(tile_size);
	} else {
		float overlap = 0.0;
		switch (tile_shape) {
			case TileSet::TILE_SHAPE_HEXAGON:
				overlap = 0.25;
				break;
			case TileSet::TILE_SHAPE_HALF_OFFSET_SQUARE:
				overlap = 0.0;
				break;
			default:
				break;
		}
		return _get_half_offset_terrain_polygon(tile_size, overlap, tile_offset_axis);
	}
}
Vector<Point2> TileSet::get_terrain_peering_bit_polygon(int p_terrain_set, TileSet::CellNeighbor p_bit) {