### 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)

bit_rect.position = Vector2(-1, 1);
			break;
		case TileSet::CELL_NEIGHBOR_BOTTOM_LEFT_CORNER:
			bit_rect.position = Vector2(-3, 1);
			break;
		case TileSet::CELL_NEIGHBOR_LEFT_SIDE:
			bit_rect.position = Vector2(-3, -1);
			break;
		case TileSet::CELL_NEIGHBOR_TOP_LEFT_CORNER:
			bit_rect.position = Vector2(-3, -3);
			break;
		case TileSet::CELL_NEIGHBOR_TOP_SIDE:
			bit_rect.position = Vector2(-1, -3);
			break;
		case TileSet::CELL_NEIGHBOR_TOP_RIGHT_CORNER:
			bit_rect.position = Vector2(1, -3);
			break;
		default:
			break;
	}
	bit_rect.position *= Vector2(p_size) / 6.0;
	Vector<Vector2> polygon = {
		bit_rect.position,
		Vector2(bit_rect.get_end().x, bit_rect.position.y),
		bit_rect.get_end(),
		Vector2(bit_rect.position.x, bit_rect.get_end().y)
	};
	return polygon;
}
Vector<Point2> TileSet::_get_square_corner_terrain_peering_bit_polygon(Vector2i p_size, TileSet::CellNeighbor p_bit) {