### 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)

break;
			case TileSet::CELL_NEIGHBOR_BOTTOM_LEFT_SIDE:
				polygon.push_back(point_list[1]);
				polygon.push_back(point_list[2]);
				break;
			case TileSet::CELL_NEIGHBOR_LEFT_SIDE:
				polygon.push_back(point_list[2]);
				polygon.push_back(point_list[3]);
				break;
			case TileSet::CELL_NEIGHBOR_TOP_LEFT_SIDE:
				polygon.push_back(point_list[3]);
				polygon.push_back(point_list[4]);
				break;
			case TileSet::CELL_NEIGHBOR_TOP_RIGHT_SIDE:
				polygon.push_back(point_list[4]);
				polygon.push_back(point_list[5]);
				break;
			default:
				break;
		}
	} else {
		for (int i = 0; i < point_list.size(); i++) {
			point_list.write[i] = Vector2(point_list[i].y, point_list[i].x) * unit;
		}
		switch (p_bit) {
			case TileSet::CELL_NEIGHBOR_BOTTOM_RIGHT_SIDE:
				polygon.push_back(point_list[0]);
				polygon.push_back(point_list[1]);
				break;
			case TileSet::CELL_NEIGHBOR_BOTTOM_SIDE: